Well, chalk the disposal of the old graphics card up to experience. Probably a good idea to hang onto old parts for at least 2 weeks, just in case.
As for options? It’s going to come down a bit to throwing things at the wall to see what sticks, if you take my meaning.
Start by re-seating the new graphics card and PEG power connector. It could be that it’s just not making a good connection, so refit it and also double-check that everything is fully - and evenly - plugged in. Also, if the power supply has it, try using a different PEG power connector - make sure you’re using an 8-pin cable for an 8-pin socket (even if it’s one of those ones that just loops two of the wires back from elsewhere).
NB There’s likely a “P4” connector on the motherboard with 8 pins. These have a different pin configuration to the PEG-8 connector and aren’t interchangeable. You shouldn’t physically be able to connect the wrong one without using excessive force anyway, but it’s worth knowing.
Next would be a software sweep. You’ve changed GPU architectures, even if not manufacturers, so it’s possible the drivers have some things working a little wonky. You can run the driver installer, select Custom (Advanced) and check the Perform a clean installation option. Or, and this is generally preferable, use a tool by Wagnard called the Display Driver Uninstaller (DDU) from Safe Mode to completely remove the old drivers before re-installing them.
After that? Well, the easiest thing to do would be to watch temperatures. TechPowerUp’s GPU-z can monitor them, as well as log to a file if you can’t watch it yourself while waiting for the problem to occur. Unless it shoots up particularly high (90°c+) than you might have to get it to reboot a few times in case it happens at a particular temperature.
There are other steps which could be taken, but I’m not 100% familiar with the software necessary to do it, or if it even exists anymore. It would require some kind of test suite which checks over the VRAM and portions of the GPU, but I haven’t seen a purpose-made program to do that in a very, very long time (mostly it’s just stress tests and benchmarks these days).