Do you get the same choppy results if you bypass the monitor? (OSSC directly into audio extractor, or directly into your LGP?)
When did you buy your PlayStation SCART cable from Retro-Access? At some point, they started using 1000uF capacitors in their cables in order to get a happy medium between the 220uF caps that the PlayStation requires and the lack of components that the PS2 requires, and that cable should, for the most part, work with both consoles. I believe you now have to specifically ask for 220uF caps or no caps if you want a built-to-spec SCART cable for one console or the other.
I don’t have any hands-on experience with Bandridge SCART switchers, but it looks like the inputs on the front are wired to AV1 on the back; so, when you have AV1 selected, you could have video from the the SCART input (assuming it has no audio connected), and audio coming in from the front.
If you want an automatic switch that’s capable of switching and passing-through both RGBS and YPbPr from SCART, you may want to hold off until the next batch of gscartsw; it will be an updated version that is compatible with RGsB/sync-on-green, which also makes it compatible with YPbPr. Pair it with something like this adapter, and it should be able to pass-through your YPbPr sources, with audio.
Do keep in mind that this switcher will have a mode that automatically converts RGsB to RGBS, completely stripping green of its sync signal, and that doing the same with a YPbPr source will make it unusable. Basically, you won’t be able to have both YPbPr pass-through and RGsB->RGBS conversion at the same time.