I agree about Avermedia, although they sell this as an entry-level consumer-grade product for gamers, and their support team are probably glad to avoid dealing with Linux tinkerers :) Now I'm savvy enough to know that an USB Video Class device, like many webcams, can be used on Linux or whatever supports this standard, and that h.264 can do interlaced video (like MBAFF) that i prefer for my source.
Ok, GC311, as most HDMI capture devices nowadays, can de-interlace source to progressive, which (in my opinion) can give mixed results, and use more bandwidth than necessary when, like me, one wants to preserve interlaced fields, even if you're perfectly right about compatibility and practicality.
Good to know about the GC311 stability on your setup, and that, through UVC, you can only select a combination of resolution and compression types (with fixed bandwidth). But I'm still looking to confirm if it can however provide *interlaced h.264 output* directly via UVC, like 1080i50.
Eventually, when you have the chance to get to your rpi4 with gc311, could you provide text output of 'v4l2-ctl -D --list-formats-ext -d 0' or 'uvcdynctrl -f' commands that list all modes ? Cheers