Finally got to it and I AM SO GIDDY RIGHT NOW!!!
I looked at your files to know what I should have expected, then, since I'm on Silverblue, I followed this guide https://julianhofer.eu/blog/01-silverblue-nix/ coupled with with Home Manager's Flake manual and finished off with installing Devbox (through Home Manager, which isn't listed as one of the official installation options for some reason), made a Python environment with it and... it's all looking good!
Declarative workflow, here I come 🤩 (and I can soon shelve all those rusty distroboxes that don't start anymore because Podman/Distrobox weirdnesses which have been all to frequent in my usage, yikes)
Thanks again! I probably wouldn't have taken the plunge so soon without your comment