How useful would investments in my Tangela solo team be in the long run?

If you already have the fighting attackers you may as well give FS/OH Blaziken a run. Are weather boosted Eevee common around you? It wouldn't hurt to catch some L30+ and keep evolving them to pad out your team. I hope your dodging is up to par since Flareon and Blaziken rely on it.

As far as rare candies go, that's kind of a tough call. I'd at least wait for Moltres day to pass and see how many you come out with before sinking into them - hopefully you'll get enough catch candy to pad one to L30 at least. FS/SA Moltres has different utility than FS/OH so the investment won't be completely lost. The real Gen 4 threat is Heatran, potentially rendering all existing fire types obsolete.

Fire's use in general is somewhat limited, at least it's challenged by other types that are more universal. After Tangela leaves there's no guarantee a raid boss weak to fire counters will be introduced. The only incoming T5 I can think of is Virizion and that's two generations way yet.

I guess we need a couple answers to give you a good response:

Are you capable of soloing Tangela already? I would assume no, but if so there's no sense doing anything. If not, what's the minimum investment you need to get there? Can you take the flareon and charizard to a break point? Can you evolve a few level 30+ Eevee to get more viable flareon?

I wouldn't use any rare candy on moltres or entei, as there's no guarantee they'll be used for anything else before they're outclassed. The Rayquaza is a safer investment IMO, even if it faces competition both as a dragon and flying attacker - it will likely be useful as a dragon attacker INTO the palkia that will replace it (your only risk is if dialga comes first, not being weak to dragon, and outclasses Rayquaza AND Rayquaza also gets outclassed on the flying front; fairly likely it will still be useful overall).

The second question is whether you need to raid Tangela solo, and how many do you expect to do. It's trivial to duo if you have a friend or an alt even (no need to play much on it, it's unbelievable what difference a low level account can make). If you're doing it out of interest but already have a few Tangela hatched, I'd possibly reconsider. We don't even really know if tangrowth will be top tier (I'm not desperate for it and I missed bulbasaur day, if you have frenzy plant you probably care even less). Those passes may be better spent on rhydon and legendaries (to get more TMs and rare candy).

Finally, that's an awfully large rare candy stack for not having anything over level 30... It's your call, but I'd consider investing more in Ray and Kyogre).

Cheap way would be to evolve the high level CD Eevees. 1/3 isnt that bad of a chance and if it was partly cloudy you should have a ton 800+

I would actually go with the Rayquaza. It isnt threatened by Palkia or Dialga that much actually, they both have lower Atk than Ray. Not to mention Ray is quite versatile as a top counter in Champ, grass AND dragon raids. I have 2 flying Rays at max lvl and another one with a dragon set. I used to have 3 dragon set Rays, but thosd two guys already take half life of Tangela with a little dodging vs sludge bomb. It will give your team a nice boost and is a viable long term investment imo.

Sludge Bomb is really tough, best strategy would be to find one that doesn’t have Sludge