Grind character levels

[SeasonedNoob]

Is there an ideal level to grind the character levels (eg. level 50 - 100)? All I've managed to find are guides to increase crew level, which is significantly easier to grind, and which you'll get naturally if you're levelling all characters to 100.

Currently using the level 30 and 75 (arena) levels to the right of starting island in dream log, but progress is slow. Not using the level 100 islands as not every character can survive at level 50 on those islands (Fine, I'm bad at some of the characters...).

Does island level even make a difference to levelling rate?