1. Islands are done, but we still have 1-2 more areas, won't get into spoilers about them. We don't know if they'll be big explorable areas or domains or just boss fights though.
2. Yea since you don't give a crap about events or whatever, there's not really a bad time to leave/come back. They occasionally have those returning adventurer campaigns where you can get bonuses and free stuff, but eh.
3. You can still get a really good polearm from fishing, but yea you missed a good Claymore. We're also getting a suited-to-Albedo sword coming up. I'm not sure if you played back during the Klee summer event, but there was a good free weapon there too.
4. Shogun hit high banner $s, and she's good. Ayaka and Yoimiya have their places but can be skipped if you aren't fans of them. Sara is cute but kinda bad. Kokomi too. Sayu was fun to play but not very good, gets better at high constellations. We're about to get 2 Geo bois, including a furry. Out of all of those, I'd say only Shogun was the "want to have" character.