Success!! My Solus installation is now complete. I highly recommend you try that wheel mod if you haven't.
So to recap, it was these two packages/commands that solved it:
sudo eopkg it -c system.devel
sudo eopkg install python3-devel
I know it's a longshot, but could that app/script be prebuilt and included in the repos? (I have no idea how these things work, so apologies if I'm saying something ludicrous).
EDIT: I can only select a single best answer, but both of them were the solution.