Alright. So seems like my issue was related to this: grub efi fails on latest nixos-unstable · Issue #61718 · NixOS/nixpkgs · GitHub (got error: symbol `grub_file_filters’ not found / Entering rescue mode when chainloading Windows manually) but it was so annoying I got mad and changed my partition scheme from mbr to gpt and reinstalling everything. Sorry I couldn’t provide more info for that Grub issue, but this was better than throwing my laptop out of the window in rage. I’m also now using systemd-boot on 20.03 and it detects windows with os-prober no problem.
Can be closed I guess?