Yeah, the cheapest good-ish solution is the ~$100 elgato camlink 4K, which will give you 4K 30fps in. It works well enough. Skip the really cheap crap, as you say.
Right now the best solution is using hardware like the Atomos Sumo, if you want multiple sources and full quality. I wish there were an easy breakout for 2-4 HDMI or SDI inputs straight through to a multi cam timeline on my Mac.
Yeah, I got back into video recently and it's pretty surprising that the "multiple video in" isn't a solved problem in 2024! Even recording "clean" video by itself is unexpectedly tricky. The only choice, on a mac at least, appears to be to record directly from the USB device using quicktime.
Seems like a definitely opportunity for someone, although it also looks like Apple are thinking of moving into the space so maybe not.
I ran into so many obscure, incomprehensible issues with OBS I just gave up. I'm sure the other two can also be coerced into doing what you need but Quicktime just works. Maybe things are different on PC.
FWIW: Most of these cheaper devices will actually be ~720p max upscaled.
https://www.naut.ca/blog/2020/07/09/cheap-hdmi-capture-card-...