Heck even moving it to another partition isn't really a re-install as it'll happily create the exact 1:1 same system based on nothing but the configuration file, change nothing but the id of the root partition (you'll have to move over /home manually, though).
And if you mess up your configuration either roll back instantly, or fix it in situ in case you already gc'ed the old stuff. It's practically impossible to get it into a non-booting state without literally ripping out the disk it's installed on (or, well, Windows messing up the bootloader or something). Even if you run unstable on the whole system every single commit on that branch is tested to not break boot and rollback.
Oh just one thing: Don't skimp on the size of your EFI partition. 100M are definitely borderline when you have both NixOS and Windows booting from it, those kernels and initrds have gotten quite large over the years and you'll need to be able to fit, bare minimum, two of both.
Realistically you don't have to if you're not constantly tinkering, but if you're changing a lot of low-level stuff without knowing what you're doing, you have the ability to break things. If you don't know how to fix them, then it's easier to just reformat. Basically it's a skill issue lol.
