Linux

do you stick with the same distro across your PC, laptop, and server, or do you pick different ones based on the device and what you’re doing?

Definitely the latter.

constantly have to look up flags for different package managers.

FWIW, you don't have to choose the distro's package manager. While it makes sense in most cases, it's definitely not a requirement. And that's where the nix package manager comes in. Unbeknownst to some, you don't have to be on NixOS to access it, simply because it's probably already found in the repository of the distro you're using. So, frankly, you can even expect that it's well-supported.

I feel like NixOS might be the only distro that could realistically handle all these use cases

FWIW, containerfiles used in conjuction with bootc to boot your OS from, do allow differentiation within a single containerfile; i.e. specific changes are only applied for the designated target. This is accomplished by virtue of a single containerfile being able to produce many (very) different container images to boot from. So, in short, other avenues exist and bootc happens to be one I know. Note that bootc doesn't (necessarily) push you towards Fedora(-derivatives). Despite being very new (and perhaps somewhat experimental), Bootcrew does provide container images for other distros; some of which have already spawned multiple derivatives of their own. See e.g. Tartaria and XeniaOS.

but I’m a bit scared of the learning curve and the maintenance work it’d take to migrate everything over.

My 2 cents: if you're interested in NixOS, just start out with installing nix on any distro. And see where that takes you ;) !