Note that some firmware upgrades will only be applied if MacOS is booted from the internal drive (NVME support was added this way to some models). I leave MacOS on the machine for as long as it's supported and dual boot other operating systems from external devices. Some can run completely in RAM if there's room, which is nice.

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).