I made a similar switch half a year ago and thankfully for me it was relatively painless. Some stuff got significantly harder to set up (e.g. getting a nice rust development environment, getting ROCm to work with some torch-based project), but once all that is done I have complete or near-complete setup instructions on how to do it again, so I am hoping the trade-off here will be worth it in the future (or I will drop nixos and move to something else if I get bored, time will tell).

For the beginners, I recommend to go with the flakes setup right from the start, here is a nice guide that you can use as a reference: https://nixos-and-flakes.thiscute.world. I followed it through for the initial setup and I don't remember having to think about channels, at least initially: I picked the most recent stable one right at the start and only updated it to another - the unstable one - later on when I wanted to get some fresh kernel version. The upgrade was pretty painless, as the channel is just the root input of the flake: change that one line, nixos-rebuild switch and it's done. With flakes I occasionally run nix flake update (+ rebuild) to get newer versions of packages (as the flake will be locked to the state of the channel at the time you install/update). If anything (well, most) of the things go wrong, just go back to the previous build while you figure out what's causing issues (much better than the Arch experience of something going wrong after the update - better read Arch news regularly 🙃).

Besides updating my configuration to add/remove packages and doing the same for development environments (btw, for getting compile time dependencies into nix-shell, you need to add them to buildInputs of the shell: https://nixos.wiki/wiki/FAQ/I_installed_a_library_but_my_compiler_is_not_finding_it._Why%3F ), I only ever use nix profile install nixpkgs#<package> if I want to just run some app without adding it permanently. After these 6 months of use, I have found out I am getting much less software/package cruft building up in my system. If I stop using something (especially a big think like a DE), I can just remove it from the configuration, rebuild and that's it. With Arch, I probably even forgot about half of the things I installed there over the years.