• log off all services you can (no iCloud, no Apple Mail,...)
• use Free Software alternative as much as possible (Thunderbird instead of Mail, LibreOffice instead of Apple Page/MS Word, Firefox or some fork of it instead of Safari or, worse, Google Chrome for which you may consider chromium based-alternatives: Vivaldi and Brave being the first two I can think of).
• Install a firewall like LittleSnitch to monitor what's going out of your Mac... believe me, there is a lot. Sorry, I forgot the name of the free alternative to LittleSnitch which is paid for and not open source (but is still a great little app).

When I was using one of those Mac with soldered on storage (I have been using Apple since the mid 80s, up until a few years ago) I also booted from an external thunderbolt SSD drive. Doing so I was pretty confident I would not be writing anything personal on the soldered-on internal SSD. Doing so, I was sure I would be able to get all my data of the system if I ever needed to sent it back to repair or whatever without hoping that the secure erase was indeed working as expected.

I recently dusted my 'old' M1 Mac and tried Ashai Linux on it. It worked quite well but I will wait for more apps to become compatible to give it a more serious look.