Powering up above level 30?

1. Yes...but I have made exceptions for certain attackers.
2. YES - what you power up and how much you power it up will depend on your local area and local gym scene - and it will change over time as the local area changes.
3. It is ideal to keep balance rather than have 1 or 2 much stronger mons, while not being able to have a full strong team/full strong defender list.
4. Yup...once you get to a point where you have all you want, and you keep getting resources, you may wish to adapt your self imposed rules and just level up anything/everything you use regularly
   4a - you may like to keep some resources as insurance against changes and adjustments, so you can react and adapt immediately.

My main defenders that I would like to sit for a long time in a gym are what get priority when powering up - the stronger they are, the longer they hold a gym usually.

1) For one exception, I power to level 31 for some really good attackers. It's the last level with the 4 candy and 5000 dust cost (so, you're getting half the utility from the stardust and candy you put in. Thereafter, you get a third of the utility for candy cost and 5/6 of the utility for dust cost for the next level, and then it worsens even more)

2. Yes, only reason why I power my gyarados and rhydons.
3. Not necessarily, only project I have in mind is a rock golem. I'm not actively looking for a bug type, they're quite useless in this meta. Not going to power my muk either.
4. Not the case for me, Level 34 and 110k dust so I haven't reached the 'too much dust to spend' stage yet.

I power attacker dragonites to Lv 31, and I max out (35.5) my gyarados and rhydon due to abundance of candies. I also have a defender dragonite at lv 31.

Other than these 4 mons, I only powered a venusaur to 30.5 by accident. Everything else is level 30 (or below)

I would alter 4) a bit. I find that it helps at feel good factor to have one or few favourite mons that you power up first after level up (and if they are available). I'm lucky since for me those are 3 of my Vapes, each one with a different move and even WP is still useful (in a remote stable gym, in fact he missed latest level up).
Keep in mind you are still new to the game and getting bored is not yet an issue, but soon enough you will feel it from time to time. And you need to have tactics against it (power up favourite mons, prestige/attack with random mons, discover more insight into the game, etc.)
Other than that your rules are the backbone of efficiency in this game :).

I don't agree, I'm lvl 32 and my experience proved that you also need to power up some attackers past lvl 30. Sure snorlax and blissey are strong defenders with high CP but if you live in my area, you would know that you need strong attackers since gyms' colors keep changing frequently. I maxed out one of my two dragonites which has DT/H (the other one got SW/HB, terrible!) as a generalist attacker and an attacker into dragonites since most dragonites in gyms around my area have SW while very few have DT (I don't like using Lapras now unless for prestiging). I also spare one of my snorlaxes for attacking since he has good IV and L/HB as a backup generalist attacker. The other attackers I maxed out are 3 vapes, all with HP, to attack rhydons and tyrants (I don't have a machamp yet despite having 2 dragonites already), 3 eggys with Cf/SB to attack water defenders or to be placed in a gym in case I already used all of my better defenders, and 1 jolteon with TS/T to attack gyarados although I won't power him up again I would recommend you to do so if you have a good IV jolteon with TS/TB a.k.a the new jolteon's best moveset (I was so sad after moves update which made thunder a very bad, if not the worst, electric-type charged move). I'm currently looking for FS/OH flareon and TS/TB jolteon but haven't got the luck to get one and I'm going to power them up past lvl 30 if I got any of them with good IV. I'm also going to do the same when I get a machamp with C/DP or a tyrant with B/SE.

I don't really understand the "never level an attacker beyond 30"-hint. Attackers are those you use activly. You don't fight for yourself with defenders. For me, it feels good to attack with a strong attacker, so i like to raise them too.

Yes, it is true, you can kill every gym with every trash-suicide-squad. You do not need good attackers at all. But if you want to enjoy a battle, be time efficient and potion efficient (maybe kill before opponents charged attack, or kill a vaporeon with one solar beam and and a few quick attacks, preserving the next solar beam for the next opponent and not having to waste it on first vaporeon too), you will want a strong attacking squad too.

Regards BB

Tyranitar and Dragonite are definitely worth it for the CP to stay on elite gyms - of course, this depends on how gyms are where you play.

I've personally always enjoyed the collection aspect and have put personal preference above actual gameplay value on some Pokemon... so I have a few 100% lv39 mon that pretty much no one else would ever consider spending dust on, such as Golduck and Tentacruel.

Nowadays, for true power I'd only bring a 90+ Snorlax, Dragonite or Tyranitar above lv30, with the proper movesets... I already have plenty of stuff in line to power up anyway.

I powered up a couple of Blisseys, and a Snorlax beyond level 30, because they are hard to prestige and if they don't have high enough CP every fool teammate and his brother dump higher CP gyrados and rhydon into the gym. This kind of stops the development of the gym, versus if some of these paper tigers end up below the Blissey/Snorlax.

For attackers, lvl 20-25 seems to be a sweet spot. Their attack and defense is already almost as strong as they are going to get. Further leveling up mostly benefits only their HP, and at this level strong attackers have a pretty good chance of being able to prestige some Blissey, or Snorlax that someone left in the bottom of the gym.

Whether to power up a defender boils down to a simple question: Do you have more defenders than you can keep in gyms? Phrased differently, are some of your top defenders sitting around in your pokebox most of the time? If yes, you do not need more, you need better defenders - power up. If no, you do not need better, you need more defenders - save up candy for evolutions.

Rather than having fixed rules, it's more dependent on your local meta and gym strategy. In some urban areas, you need 3100+ CP defenders to get in the lowest spot in gyms. In some rural areas it's common to have 2000 CP defenders or lower, towards the bottom of gyms. Most areas fall somewhere in between. If you need defenders above level 30 to hold high places in gyms, then power them up. If not, then save your stardust for others (including attackers).

Powering up attackers depends on how much stardust you have to spare and how much benefit you'll get from powering up, which is too complex to summarize with simple rules. For example, I powered up my attacking B/SE Tyranitar to level 33. I chose level 33 because that will allow me to fight up to level ~35 Blisseys with Bite rounding to 5 damage instead of 4, which is the strongest Blisseys I've faced in my local gym meta. And getting that 5 damage from Bite instead of 4 will reduce battle time by 10-15 seconds, which is very important against Blissey.

However, I only powered up my defending Tyranitar to level 30. Level 30 is ~3100 CP for a good Tyranitar, which is more than enough to hold a near top spot in my area gyms, so no need to waste stardust and valuable Tyranitar candy by powering it higher. In this case, it makes sense to power up an attacker moveset beyond level 30 and keep the defender moveset at level 30.

I simply max every very good and viable attacker or defender. If I attack a lvl 10 gym, for example, several maxed attackers such as Snorlax, Dragonites, Gyarados, etc, and some other dedicated attackers/defenders, make it potion/revive efficient, and much more fun. That's my personal strategy, but is admittedly costly to do so. Rewarding though.

i would pause at level 30 but consider your specific needs

are you a minority team
are all your defenders going to be turfed out in one night regardless and will you be regularly taking out level 8 gyms
if so you need strong attackers that can take out multiple mons at minimal cost.

are you in the dominant team were the important thing is a strong defender (level 30 blissey and snorlax)

are you in an area with lots of shavers in which case you need a high cp maxed gyrados/rhydon/dragonite which can do double duty as attackers in some instances.

go to level 30 then if you find you are struggling to get into gyms power your attackers, if your are struggling to stay in gyms power your defenders.