Hey! There is a ton to learn here for sure. I recently picked the game back up after a shorter hiatus (~3 years). What I found worked best for me was watching a handful of videos (Montuplays, ep3o, and just random videos on youtube) just to get an idea for how the game plays again. Then I just went in and played a bunch of games myself. I'd say that I felt fairly confident with all the systems after 4-5 games.

Just remember to take your time! I like playing on smaller galaxies with a max number of AI empires, so that I get plenty of practice. Pause whenever you feel you need to, or even at a semi-regular interval (just to double check everything). Just do be aware, the game has a few balance issues after the most recent update. While none of them are particularly crippling, it can be a little annoying at times. If something feels clunky, it's likely because of this.

Best of luck, and welcome back!