Just a final note on this (for now): After a bit of digging, I found the
"Domesday project", which is very close to what I'm looking for. They designed a 10bit 40Ms/s digitizer for continuous streaming (including a FPGA and USB3.0 board) with the main purpose to digitize the RF signal directly from a laser disc head and do the entire RF to Y/C to RGB decoding in software. The software project has a fork for decoding VHS RF (called
VHS-decode) which produces very decent results already, including color, and is in active development, so I don't have to do it from scratch. It seems to be the superior approach compared with digitizing the composite or S-video signal, so I will give it a shot sometime when I'm free enough. There is no Hi8 support yet though (which would be more important than VHS), but that's a matter of software adaptation once the RF is recorded from the tape.