I have issues

Hey everyone, I bought this 4Runner back in February 2025. It ran like a top after my son and I changed the timing belt, water pump, and all of the timing component system. On July 4 weekend, I installed a traffic flasher system on my headlights. It did not work like I thought it would, and when I tested it, it made some sounds over my start relay, or it was making its own sounds. Probably defective because I bid on it on eBay and it was used. Nonetheless, I had been doing a soft reset on my computer, and I installed new Denso high-tension wires. When I went to start it up, it struggled a bit but then started. Then it began backfiring and threw a P0300, P0302, P0204, & P0306. This week I changed out the valve cover gaskets, rebuilt the 6 injectors, and put it back together and started it up. It threw the same codes; no other codes just those.
Any Ideas?

 

300's codes- multiple misfires - 204 code is injector circuit- cyl. 4. Not exactly sure what would have brought this on after running well.