This seems like it should be supported by looking at the first couple Series of FH5:
Series 1 of FH5:
Four returning cars, one New-to-Horizon (This included the Zerouno, the McLaren F1 and the Tachyon so not really that bad of an update outside of making the AE86 a FOMO car for no good reason)
Series 1 of FH6:
24 returning cars, one New-to-Horizon, two New-to-Forza--though this is inflated by the massive number of FOMO cars in the playlist
Series 2 of FH5:
Seven returning cars, one New-to-Forza (A very stacked update IIRC--two returning Ferraris, three Lambos, including one New-to-Forza in the Aventador SVJ, the 8CTF, and... the two Peels.)
Series 2 of FH6:
One New-to-Horizon, one New-to-Forza, three returning cars
Series 3 of FH5:
One New-to-Horizon, seven returning cars (Outside of the Donkervoort and the Zenvo, I don't really remember this update setting the world on fire exactly. The New-to-Horizon being the Celica SSII didn't help matters, especially since it was the last car I needed to get to complete the collection once upon a time and I had a hell of a time getting it!)
Series 3 of FH6:
Three New-to-Forza, five returning cars (counting the Cosmo being made available in this Series)
Series 4 of FH5:
Three New-to-Forza, one returning car (Mostly loaded down with Chinese cars and a 996 911. I remember the NIO being interesting, but the Wuling and MG3 were unremarkable at best.)
Series 4 of FH6: (We are here)
One New-to-Horizon, two New-to-Forza, two returning
And for the hell of it:
Series 5 of FH5:
Three New-to-Forza, two returning (Perhaps the closest thing to a 'baseline' update I can think of. I can't really say much about it.)
It does seem like it makes sense, though Series 1 through 3 last game were made up of a lot of returning cars, with only three actually new cars coming in, while Series 4 opened up the faucet more. The Car Pass carried a lot of the New-to-Forzas in FH5 around this time too, though it actually did have one New-to-Horizon in it at this point while FH6 currently has two.
Honestly, only one Playlist update actually complemented the Car Pass well at this point, and that was Series 2 basically being loaded with great Italian cars, returning and otherwise.
Oh, and since I got curious, the first Automotive Excellence update of FH5 consisted mostly of expensive Autoshow cars and only three cars getting added outside of the Car Pass--one returning, the rest New-to-Forza. Maybe set expectations a little low for British Automotive, if history serves us.