". . . assuming activating Guest User would afford some kind of protection . . . "
Not really. Anyone who knows enough can bypass it as well any other accounts you have installed. You could set a firmware password and/or activate Filevault:
Set a firmware password on your Mac
Encrypt Mac data with FileVault
Both of which might be a step too far for your skills and needs? No offence intended. Besides replacing the battery does not require the service technicians to log into any account on your computer. It's not part of that kind of repair. They can do everything they need to do with tools provided in GSX and Gateway Manager.
However if you want to be absolutely certain then 'clone' your internal drive to an externally attached one, reboot from that and erase the internal drive. That way it goes to the service centre with absolutely nothing on it. Not even a system, yet you have all your data, OS and applications exactly as is and safe in your hands.
When it comes back from service, attach the external drive, 'clone' the external back to the internal and reboot from that. Everything is back as it was before. Furthermore you'll have a complete and identical back-up ready to use at any time if and when anything else should happen.
Small form factor (2.5" SATA) hard drives are cheap enough and you only need one as large (better still larger) than the one you currently have. External USB drive caddies are also ridiculously cheap. You can google these for yourself.
For cloning I use:
https://bombich.com/download
Does not cost much, been using it for years and a 'must have' in my view.
Good luck
