I'm returning after a long break. Should I create a new account or use my old one?

Just use your old one. Any and all starter benefits are also given to older players.

Any guide thread I could read to refresh my memory?

Gamepress
New Player Hub: How to Girls' Frontline

Should I come back to this game?

Why did you leave in the first place?

If you only care about the story there's a selection of materials to choose from.

Everything else, please see the guides.