Did you try https://devenv.sh/? It uses Nix under the hood but with an improved DX experience. I haven't used it myself personally since I find Nix good enough but I am curious if you would still choose mise over devenv.
Ultimately, we might still end up moving to straight Nix flakes, just not sure yet.
Devenv's maintainers are friendly and responsive when it comes to contributions from downstream, and like 90% of the devenv repo is plain ol' Nix code written in idioms that are common in the community.
I mention it because my team has hit some papercuts as well, but I've been really happy with how easy it's proven to address them. :)
Today though mise has so many other great features, I would still choose it over devenv or devbox.
I'm curious what some of the mise features are you like.
The ubi backend means I can use nearly any binary published on GitHub without needing to worry about a flake or nixpkgs. Just get the binary directly from the author. Same for many of the other backends https://mise.jdx.dev/dev-tools/backends/.
Tasks are very powerful, they can use dependencies, flags, args, completions, and watches. Also can be defined either as strings in the config or point to shell scripts or even python/node/whatever scripts.
The shebang trick is very fun for writing portable shell scripts https://mise.jdx.dev/tips-and-tricks.html#shebang
The fact that mise doesn't depend on nix is both a blessing and a curse. I have less tools available and I don't have the power of a package manager, but on the flip side I don't need to deal with the idiosyncrasies of nix.
Tasks sounds similar process-compose which is bundled into Devbox. I'll have to read up more on tasks though to see if that's an accurate assessment.
Nix is definitely a double-edged sword... One thing I like about Devbox is that it keeps Nix mostly (!) out of sight, mostly unless I want a binary from a GitHub release :).