I make no such claims as to what’s best I’m just reporting from my own LG C9 experience.
For me N64 doesn’t look better on the FM it looks the same (or the same with some extra added noise). Why would you use pass through with the N64? Several N64 games (Pokemon Stadium) REALLY struggle on the FM too because it takes so long to go from 240p to 480i.
I can confirm NES RGB works on LG C9 without the need of a dejitter.
For PS1 I’m not really understanding what you mean, the FM doesn’t support optimal timings anyway, so just use OSSC in your favourite generic mode and it will perform as well as the FM will it not?
Again a lot of PS1 games (Micro Machines) are unplayable on the FM because it takes so long to go from 480i to 240p, Micro Machines for example is at least tolerable on the OSSC since it only shifts resolutions in the pre-game menus.
I would say PS2 is the console that most benefits from the the Framemeister, it really shines for PS2 and easily beats OSSC here with its excellent game-optimised deinterlacing.