Hi VirusAM,
I have similar system with yours and also just upgraded from Rift S to Pico4. It runs pretty smooth now after I changed router from dual-band to tri-band Wifi6.
It might be not the identical case to you. But should worth you to consider the lag issue might be caused by network since you 've tried so many options.
Tether via type C should be the best, while it's a bit tricky to set it up with VD. You might have one from Amazon to try it and could return if it won't solve your problem.
If you have chance to try it, you should assign one band (one of 5G, or 6G) dedicated for headset only.
It's so great to wear headset without tethering meanwhile keeping wireless connection about 1200mpbs. BTW I am using Ultra/H.264 w/o buffer/SSW. Also tried Godlike, still works fine.
Good lucky and hope you could enjoy your new headset eventually. Pico4 has much better clarity v.s. Rift S and ... it's so damn cheap. Believe it would be proven just config trick for you.