Is this something that OnlyOffice could solve? They aim for strict compatibility with MS Office IIRC and it’s been working really well for me.

Yeah, I get that. I was daily driving Guix for quite a long time and really enjoyed it. As I understand it shares a lot of code with Nix. It’s just been a bit hard to integrate with a lot of the software I run due to it not being compatible with the traditional filesystem hierarchy. This is obviously a selling point for Nix/Guix as it frees it to try new ideas, but makes it harder for me to run my music production software for example, which I can’t run in flatpak officially and since it Just Works™ on Debian, I’m happy with it. Maybe I’ll get into it sometime again as the community seems to have grown a lot, and I’ve looked into running Nix on top of my Debian install.