I finally fixed my network issue! Turns out it was NI LabView services causing some sort of problem. Funny thing is I found the solution purely by chance, browsing r/forza by new and noticing someone with a similar issue. Someone linked them to another post that was posted last week, one that I'd commented on, that had been updated with a solution and it just so happened that I also had NI software installed on my PC (about a decade ago!), mad coincidence.
So now I'm one point shy of the Aston Martin prize car and have accidentally stayed up past 4am playing online with real people, most of that was just doing drift runs up and down the volcano. I did one PvP (finished 2nd) and one PvE (won) tournament and tried two Horizon Arcades (definite connection issues there, players kept disappearing), also noticed my performance is significantly worse online than off with frequent stuttering. I'm hoping that'll be patched up in time.