Mosfets get Very hot at relatively low power drop(As E load)
I don't know what else is wrong, but  you're missing a connection on the gates of Q3/Q4. Decent chance it won't be stable. Check that with an oscilloscope.
I would estimate your 30 * 50 * 115mm heatsink to have \$R_{\theta}\$ of perhaps 3.5 or 4°C/W so 20W would result in a heatsink temperature of 95°C to 105°C at a 25°C ambient.
The transistor cases, and the junctions will be even hotter, of course. The &quot;75W&quot; Aliexpress load definitely expects the user to add a heat sink and a fan. With a powerful fan you can get a much lower \$R_{\theta}\$ compared to natural convection.