Returning Player Advice

Hellooo I recently got the itch to log back into my old account and it's been a long three years since I played. To date myself a bit, the last major feature I remember was the start of the egg system. My current aces are the Sygna Suit Kanto Trio at 3/5.

It's been a bit overwhelming to learn all the new features so I was wondering what the best way to go about being reintroduced to the game is. Would also greatly appreciate any advice on things I should be looking out for to be caught up to the meta.

Also the completionist in me is wondering if there are still units locked to specific exclusive events. I know James and Jessie are locked for new accounts but are there anymore pairs?