It used to be when I loaded a bunch of clips into a project, I'd just leave it overnight and it would finish Analyzing and Transcoding. But it seems that now (where "now" means sometime in the last year) it stopped rendering when I'm not at the computer.
My computer is an iMac 5K from 2017 running Catalina. The Energy Saver settings are "Prevent computer from sleeping automatically" is checked, as is "Wake for Network Access" and "Enable Power Nap".
Turn off Enable Power Nap, also turn off screen saver. Make sure there is no timeout whatsoever on screen activity, whether for security, power savings or anything else, including screen auto-lock timeout. Turn down the brightness. Let it run overnight and see if that makes a difference. When you check in the morning, the last window config should still be up.
Despite making those changes, if the transcode seems seems to have halted, open a terminal window and type "last reboot". That will give the MacOS reboot history. If the machine somehow encountered a fault and restarted it will tell you.