The beep codes are hard to figure out, but if I understand it correctly, what you describe is:
1 beep - DRAM refresh failure.
(You see, usually the pauses between the beeps in the pattern are only 0,5-1 second long. The pauses between patters are 1-2 seonds.)
I would try this:
1) BIOS factory reset.
2) Replace the BIOS battery. (If you do not have on at hand, try other solutions first.)
3) Reseat the RAM, move it to another slot.
4) Reseat the GPU, clean the PCIE.
5) Reseat the PSU connector.
6) Try another PSU. I have not checked, but 400W should be enough.
Your motherboard has 2 RAM slots, so you can try to put the stick in the other one.
I would try this without the GPU, so you know the RAM works in the other slot.
(Some motherboards require SLOT1 to be populated in order to work.)
You have been using integrated GPU for some time right?
That means the PCIE slot has not been covered/protected by inserted PCIE card.
There is a possibility of it being filled with dust. That might cause issue. If you have can of air, blow it out.
Replacing the CR2032 BIOS battery is a good idea, it can indeed cause all sorts of issues.
Although they can last longer, they should be replaced every 2-3 years to be on a safe side.