Modern capture solutions are digital, so you'll need a way to digitise the output of your 286 or 386.
I use an OSSC and a Magewell Pro Capture 4k HDMI, and have no issues capturing anything, be it 70hz or weird resolutions. It's an expensive solution, but previous Magewell cards would work the same, you can find them second hand for 100 bucks or so, and offer up to 1920x1200@60hz without much hassle (I replaced my original Quad Pro Capture because I was moving to 1440p). Of course, at that point, you can also simply purchase a VGA capture card.
For earlier graphics adapters you'll need something that converts them into something else, like Serdashop's MCE2VGA for Hercules, EGA, CGA and MDA.