Tbh I don't want much for the PC, only a new psu, maybe a new mobo, a corsair cooler so not a great deal.
Too much wild speculation from observation not tempered by how hardware works.
Original post said a motherboard could not see the GPU. A d6 code says the motherboard could talk to a GPU using video mode 7 but not in other modes. Now a relevant concept. A defective motherboard can still talk to some GPUs just fine. A defective GPU can work on some motherboards. One PC trace on a motherboard routed too long can cause some GPUs to fail and others to work. Swapping says little other than some item can sometimes work fine.
Because so many computer assemblers have no electrical knowledge, then we tell them to install a power supply at least twice what is really needed. Most computers do not even consume 300 watts. So we tell you a 600 watt supply is required. Most computer assemblers must first learn to overwhelm their fears to be able to use a meter. That is just too difficult. Easier is to just say 600 watts are required.
Is the PSU sufficient? Watts says nothing about quality or integrity of that supply. Using a meter would have exonerated or accused something in the power system as defective. System that is more than just a PSU. Without those numbers, then nobody can say anything useful about power ... other than make sure the separate power cable from PSU to GPU is properly connected.
Does not even matter if a fan spins. Defective voltages will still spin fans. That is obvious to a fewer who solved problems using a meter. For example if that PSU to GPU cable is missing, a fan will still spin. And voltage from the PCI slot is big time reduced.
Clearly, a 1000 watt supply solves nothing. At 600 watts, is your computer also toasting bread? If not, then it is not consuming anywhere near 600 watts. But what each voltage is on each colored wire is critically important. That means a meter. Otherwise many GPUs may perform strangely - work today, fail tonight, work just find in the shop, etc.
Apparently your video card is incompatible with that motherboard. To say more means measuring voltages with a meter (a likely reason for failure and intermittent operation because the defect existed months or years ago). If a BIOS (no Operating System yet loaded) cannot put up text, well, we view things you do not have equipment for. Once those voltage numbers are seen by the fewer who know this stuff, then your know that slot on a motherboard is incompatible with that GPU. Nothing more.