That's all the symptoms of a dirty head or possibly a worn out one, but if it was fine before the cleaning, it makes me wonder if dirt was possibly introduced to the heads in some way.
Betamax has been my least favorite to work on in terms of getting tracking to work appropriately (I've gotten 3 out of 4 SL-HF900's going), but this doesn't really look like a tracking issue since the snow/comets are evenly distributed. I presume the noise doesn't change when you wiggle/rotate the output video connector at the player as it plays? No electrical interference that it could be picking up from nearby devices? This kind of looks like what you'd see back in the day when running a vacuum cleaner and watching over the air TV honestly.
But anyway, my guess is the cleaning process introduced dirt that wasn't originally in a place causing a problem, or there's the "stick-tion" (sticky friction) that can occur with betamax heads as the upper drum surface in particularly gets shiny from being polished by the tape going by. We kind of got lucky that VHS and spinning drums (rather than spinning head discs) were more prevalent and acted more like an air-bearing above the spinningdrum surface rather than betamax's dragging the tape across a long stationary surface as is it gets read.
Not that it matters, but betamax could have used an M-Loading style like VHS did. They actually made a betacam deck that does it decades later and they use the same form factor tapes as betamax:
https://www.youtube.com/watch?v=EbUPfzZ1aEM&t=1370s
What was your cleaning process specifically? Is the top drum shiny at all? There's a process for de-shining it, but there's further risk to the alignment in my experience.
