Just wanted to give a little update.
Finally had some time to get into the printer and check the connections. They all felt tight but I loosened and retightened them. Then went through the upgrade process to 3.3 again.
Now it works! Not sure what it was exactly but in any case it's heating and not crashing for now.
Still want to make a new wire loom for the hotend and have some wire on
order. If there are any electrical issues hiding in there somewhere hopefully that will straighten them out.
Thank you all for you help!