Still, without specifics on all the components, can't say whether you have missed anything. I know it seems obvious, but we don't have that information.
I can get a 450W power supply from all kinds of suppliers. Some will be up to date and support the latest ATX spec, some will be older designs that may not get along with the latest hardware. Whether it is modular or not. You would be surprised how often people forget important details when putting power cabling into a system.
As an example, even the cable kit I bought for my system had a loose wire that I had to verify and re-insert. But if it is non-modular that eliminates that as a factor.
DDR4, good, 1x8 GB, 2x4GB, voltage? Speed? Model? 1st generation Ryzen components were known to have memory compatibility issues, if the board has an older BIOS on it could easily be the problem.
If you have access to other systems, test components. See if the GPU works in another computer. Test the memory, if possible.
Are any components used? Like the power supply?
When trying to get a boot screen, unplug everything you don't need.
PSU, ATX and EPS power
CPU, CPU fan
Motherboard
Minimum amount of RAM, try each stick if more than one.
GPU in this case.
And the monitor.
Leave drives, peripherals, front panel connectors, everything else unplugged. Start the computer by shorting the Power Switch on the front panel header. Or shorting the standby 5V+ line on the ATX power connector to the nearest ground (usually pins 15 and 16 or 16 and 17)
If it doesn't start then, something is fundamentally wrong. I would suggest removing it from the chassis and doing the same test. If that doesn't work you need to start looking at components.