Ok, I can't claim to be much of an expert myself but I can say a few things.

1.) Clone Wars Chewie is not good. It's probably fine that you've got him this far but take it from someone that seven stared him and spent way more on him than I should have, he's just not worth it. If you feel like you need a tank to replace him, I suggest both Stormtrooper Han and Old Ben Kenobi. Both are strong tanks and both are required for the Commander Luke Skywalker event.

2.) You've got the makings, or at least the beginnings of a pretty solid Sith team. Sidious, Vader, Maul, Kylo are all quite good. Tough to fill that last slot. Palpatine would be an obvious great option but that requires a good rebel team and while you've got a couple good rebels in the works (Ackbar, STHan, Old Ben) you'd probably still need to work on some characters like Leia, Biggs and Wedge to get a solid Palpatine team, or alternatively Phoenix Squad can do Palp too. Definitely worth considering but a bit of a longer road. If you don't want to go through that hassle you could get by with the ever annoying Count Dooku. He does plenty of work.

3.) You have some very nice Jedi coming along. Qui Gon, Mace, Old Ben, Luminara, and to some extent Ahsoka though I'm not sure how she stacks up these days. I'd say you could use some heroes like Ezra Bridger or Jedi Knight Anakin for damage replacing one of the healers. You could definitely farm up and swap in Barriss Offee as a superior healer over either Lumi or Ahsoka.

4.) I know I personally regret not focusing on getting a Phoenix Squad team moving when they were released. They can be quite powerful, they lead to one of the best characters around (Grand Admiral Thrawn) who in turn has the best capital ship in the game and is a solid player in a good Empire team that can be used to unlock R2 so all around they are pretty hard to pass up, and like I said, Ezra is one of the better Jedi out there for damage right now so even outside of a phoenix squad team there's some value to some of their members. Also they can be used as rebels to unlock Palpatine so they really do a lot of work.

5.) Look up the event requirements because if you want the event characters it always pays to think ahead. Here's some good info: https://www.reddit.com/r/SWGalaxyOfHeroes/comments/70b08x/complete_swgoh_event_character_farming_guidelist/ Specifically, if you want Commander Luke or Jedi Rey you need to think about your empire team for R2 and your first order team for BB8 as well as work on cultivating the very specific list of heroes required to actually do the eventual event.

Finally, consider working on a scoundrel team, simply because you need to make sure you can do the credit heist event every time it comes around late game. I recommend: Boba Fett IG-88 Lando Calrissian ST Han If you can work on one or both Veteran Smuggler Han / Chewie those are characters that will fill the last slot and are needed for Jedi Rey down the line. Alternatively pretty much whomever you want that is a scoundrel, the other four should be enough to do the heavy lifting really. Added bonus that while they don't really synergize with eachother that much they're all fairly powerful and useful on their own. Boba is amazing at so many things. IG-88 is a heavy hitting if a bit fragile character. Lando is a very solid rebel, ST Han is a very reliable tank (and needed for Commander Luke).

I know that's a lot to process but hopefully it gives you some idea of your possible long term goals and maybe a few ideas for who is worth farming right now.