I think epics and legendaries will be the best way to improve your collection effectively. Most of them have very strong and unique effects.

Leveling up your archons along with some key units will also improve your winning chances by quite a bit. However remember that you can also rent higher level cards for a season whereas with epics and legendaries usually one copy at lvl 1 will already be a big improvement on its own.

Ranked is definetely the way to go, competition will be tougher but you want as many of the reward cards as possible. If you go that route you need to put some of your budget towards SPS to stake on your account, it'll give you an important multiplier on how much glint you earn!

Sadly, almost all of the foundation cards are severely underpowered so they won't serve you too well in the long run.

At the same time it's definetely a slippery slope to start investing in the game 🤣
For one thing, like you said, there's only so much time in the day and it's a long process to submit all your battles. Some days it feels wasteful to let your cards just sit there ahah
I think we've all been there..

Moreover, once you start investing in the cards you'll see that there's virtually no limit on how much you can spend in order to perfect your collection. Both in terms of variety and of levels. So definetely try to come in with a budget and some goals.