has the system been tested in it's entirety? everything include case, psu, all connections on. not just separate cpu and gpu test.
that's why ppl recommend buy mb, cpu, ram and psu all from one shop. that's the minimum to boot and test to confirm working. shop will use own gfx card to test if you want before leaving.
to recap,
cpu/ram prime95 no issue (also ok from previous usage with igpu)
gpu no issue in another shop (can boot and display, your problem is no display or artifacting)
bios is latest version
actual details of whatever tests or diagnostics done are sketchy at best. what you really need is to swap around all components to determine the fault.
combine cannot display or got artifact or unstable. old gpu also no display.
artifacting mostly due to "electrical" problems causing data error and corruption, too low voltage, too high voltage, bad contacts, failing connections, electrical leaks, etc.
assuming mb and gpu are working,
1. psu not delivering rated power over 12v
2. gfx not seated properly
3. failing pcie
4. internal conflict with pcie addresses
5. mounting problem
also, your ssd is on nvme(pcie) or sata? play around with the configuration to see if it works.