Need Help Troubleshooting My PC Build

Hey everyone! I usually just hang out in the background but I've run into a problem that I can't seem to fix on my own. I'm a novice at building PCs — I've only done it once or twice before — and I thought I could combine some of my older parts into my old case to sell it as a 'ready to buy' machine for someone starting out. However, despite doing a lot of research and having used most of these parts before, I'm having issues. I'm starting to wonder if the parts are outdated, incompatible, or if I'm just missing something obvious.

Here's what I purchased new: Corsair CX650 80+ Bronze Non Modular PSU, Samsung 990 EVO Plus 1 TB, Thermal Grizzly Kryonaut paste, and a USB 2.0 Splitter.

The parts I've previously used together include: Gigabyte Designare z390 ATX (LGA1151), Intel Core i7-9700K (3.6GHz), G.skill Ripjaw 2x16GB DDR4-2666, an EVGA CLC 240 liquid cooler, a Gigabyte Windforce GTX 970 (4GB), a WD Black 2TB HDD, NXZT RGB core fans (one F240 and three F120s), and an NZXT fan/RGB Controller. The case I'm working with is a Corsair Obsidian 750D Airflow Full Tower.

Initially, I managed to get into the BIOS, but when I tried to save and exit, it just kept looping back into the BIOS. The CPU and motherboard combo needed an update, so I went ahead and did that based on advice I received. However, after the update, the VGA light was stuck on red. I switched to a backup GPU, the MSI AMD Radeon RX 6600XT, and while the VGA light went off, now the boot light is on red instead. I've already cleaned everything up, reset everything, and changed the CMOS battery. I'm totally stumped and could use any insight or suggestions if you have them!