it may not be the best quality option, but for interim use you could get a basic usb analog capture card (such as this) which just shows up on your system as a webcam. they seem to work ok as long as you don’t need to do a lot of post processing on the images (they do add compression).
ultimately it seems that getting a decent HDMI capture card and a good upscaler is the best solution going forward since there seem to be more folks on the job making hdmi capture work on newer machines. i personally use a v4ex mixer to upscale to hdmi but there are a bunch of standalone ones as well. (side note, im still on mac os mojave for related reasons but i have older hardware so can get away with it.) i believe some of them are listed in the wiki section of this forum. additionally there may be more capture devices listed there as well.
try these: