Am I playing too resource efficient?
I always have dedicated a lot of thought towards using PoGo resources conservatively. My playtime has been going down lately because of work and weather.
Now I have more than 3.5 million dust and, what's worse, 540 rare candy in my bag.
I save them up for Gen 4 legendaries.
My question basically is: am I missing a nice to have investment in my legendaries?
I have
IVs 93% +
2 Mewtwo Shadow Ball lvl ~37 +130 candy
1 Groudon and Kyogre lvl 40 + 180 candy each
1 Moltres lvl 40 + 66 Candy
1 Rayquaza ~ lvl 35
1 Raikou lvl 40 + ~ 230 candy
1 Latios lvl 34 + 60 candy
1 Zapdos lvl 34
Would you recommend any other legendaries for me?
Honestly I wish Niantic would stop stalling Gen4 legendaries + give them better moves.
Answers
It looks like you did everything right not to spend much since they are coming back with new moves anyway. BUT to Niantics credit they didnt overpower the already best Pokemon yet (no better moves for Mewtwo & Dragonite, and I really hope Groudon & Kyogre dont get them either)
3.5Mio is a lot of dust. I'd for sure max out the ones listed. You already took them so far.
A second Kyogre maybe?
Raikou? (You already invested the candy)
For your bag problems I would put candy in Kyogre. That is never wrong (doesnt matter if he gets the new powerfull move or not). I'd always keep ~260 candy to insta max one more though
Other than that Lick or SC/SB Gengar is the shit after the CP rebalance.
You are missing some investments. Rayquaza's signature move is flying-type, so there's no reason to be scared of maxing it for use with DT/OR. SB Mewtwo is also great and a pretty safe investment right now, especially since gen 4 has so many psychic legendaries for it to fight. Whether maxing the two or making a third one is better depends on what the third one would replace in your raid team.
I would not recommend any other legendaries right now, but Kyogre is a great dump for rare candy for long term storage. Lick Gengar is also a great non-legendary investment right now, 3.5m is plenty of dust to spend some on counters that you will use for months during the following year.
Rayquaza and Mewtwo are good generalists, plus specialists for legendary raids to come in gen 4, so no risk getting wrong here.
But what then? If you feel no other legendary deserves resources and you don't need more, how about useful rare non-legendaries? Metagross? Tyranitar (debatable rarity)? Others you like/need?
Gengar is also great and I think Breloom has bright days ahead of him.
I have enough candy for all relevant Pokémon, so that I don't need to put rare candy into them. I still have a solid amount of Tyranitar candy to feed to the current ones and after community day and loads of trades I feel confident that I won't need more Metagross candy.
But I will power more Tyranitar, Metagross and Gengar. (Once Gen 4 legendaries are released I will put a lot of candy into them)