Just bought one off eBay with the 20 % off.
It’s got a years guarantee and I’m fully expecting it to go wrong.
What’s the best way to keep EVERYTHING backed up to make it as easy as possible to go back to my Galaxy S4?

Settings / General / backup and reset.

Turn google backup on and ‘auto restore’

You can backup photos as well if you use their Google photos app.

Good plan, as a common failure is bricking themselves, mine did when about 15 months old.