Power up beyond 30

No don't regret it at all. Powering past 30 helps a lot w soloing lvl and increased damage in other raids. It's fun having max or near max Pokémon as well. If you play long enough then u will have some solid lvl 30 options just from evolving so in order to improve your attacking ability, you will need to power up some of ur best options.

You can only use 6 at a time. Max em out.

Maxed around 20 now. Don't regret one.

I have taken 7 to 40 so far with my 2nd Ttar at 38.5 just waiting for the required candies. I also plan to eventually take a Lugia, Zapdos, and Moltres all the way; they're currently at 30.5, 34.5, and 35 respectively. The only thing holding me back with them is lack of rare candies. No regrets at all.

I have 42 that are powered past level 30, about half of which are level 35 or less. They were mostly maxed (for my trainer level at the time) to sit as high as possible in the old gyms.

My best blissey, snorlax, tyranitar, dragonite, vaporeon, raikou, and rhydon are all maxed to level 40. The rest are somewhere in between, either to hit certain break points, or just because I wanted to.

If you don't power anything up beyond level 30, you might as well be at level 28. That's the bottom line. Diminishing returns kick in, but a level 36 Dragonite is still stronger than a level 30 Dragonite (unless the IVs are atrocious). And if it takes you a little more dust to get there per CP point gained, so what?

No regrets

I enjoyed taking a Venus to level 37, Victreebel to level 37, and an arcanine to 33.5 in old 10 stack days for a little variation (left dragons at 27 and 28). Recently powered Victreebel to 40 (100 IV) just because. I would not have changed any of that.

That said, I sunk a ton of dust into Eggs (7 >L30 up to 39), Rhydon (5 > L30...33.5), and Vapors (9 >L30 up to 39.5). Almost all of that was old gym system power up dust investment.

What did I get for it? I collected 100 coins per day (actually probably averaged 100/22 hours) from last thanksgiving to the gym change because of it. So, while I would love to have some of that dust back it served me well and continues to do so.

Eggs great for Machamp, vapor and Omastar solo raids. Vapors were good for arcanine solo raids (curiously especially wild charge?) and ttar, moltres, Entei, etc... until other “supreme” counters were powered. They still are useful in Ho-Oh raids, too. They’re big enough to take a solar beam and can get off several charges against non-SoB variants.

Maxed 11 mons to 39. No regret. I will feel tired and boring if I just lv up my favourite mons to 30.

This is what the stardust for, just pick the loved one to pick up, your own choice.

I don't think that you will ever regret it; however, the best use of stardust is to figure the breakpoints if the Mon you are likely to raid with and power them to those breakpoints.

Stardust is the main bottleneck in this game and Gen 3 is just around the corner. I have around 650k and am thus cautious about even powering up to L30, but you have two million, so...