Dual-boot rails upgrade strategy that I’ve seen mentioned a few times.
Synvert helps automatically convert syntax for some rails updates, and a few other gems.
Some good notes on code organization from Jason Swett, the rails testing guy.
ActiveAdmin has a lot of mindshare, but I use it extensively and hate it with a passion. Between the obnoxious DSL, the shitty html-in-ruby that is Arbre, and the amazingly bad performance hit reloading a large admin site from scratch on any code change in development, cannot recommend.
Administrate I’ve tried in a smaller app, it has a bit of magic API for working off of common templates (so for defining fields for the index/show pages, etc), but easily allows you to generate whatever view you need so you can override it with simple erb.
Avo is new option, and is paid for extended features. Runs Hotwire on the backend, but might be worth looking at. Has a useful default design for filters/actions that doesn’t take up a lot of space from the main interface screens.