How easy is it to keep a version of MacOS alongside? How long could I expect a new battery to last on a charge? I was thinking of buying a new MacBook M1 but I can’t afford the crazy price, and it’s been a nightmare to manage the OS updates.
Don't know about having a MacOS alongside but if the disk is small, you might not want that anyway. And I presume you could always go back to MacOS later.
I read this is also an experience many Windows users have had when setting up dual booting systems with Linux.
At this point I think it's generally a bad idea to setup a dual-boot system with Linux, better to use it exclusively.
But if you do need to dual boot I remember doing it a few times and the refind bootloader was very nice: https://www.rodsbooks.com/refind/
Security updates are still happening.
I'm running MacOS Ventura on a Macbook Air 2013 and Macbook Pro 2015 with it. They work reasonably well and get patch updates via official updates when released.
When the model is no longer supported with MacOS updates, I wipe the disk and install Linux. Sometimes I'll keep a copy of MacOS on an external drive if I need to access or troubleshoot specific features of that model (like digital optical audio before support matured on Linux).
At least the restore sector is kept, and if you decide to roll back/sell, you can restore macOS.
Although I used it for years, and also having about 14 years of GNU/Linux experience, I was very comfortable.