I'd probably throw in an incredible GPU and try to ride the 10400F as long as you can. I did that with a last-gen 4070 Ti Super and a Ryzen 3600 which is similar to a 10400F and it was pretty solid. I seemed to usually float around 100-120FPS @ 1440p, though I know some titles do exist out there that you won't have quite as good of results with. Did it hold it back? Yes, but if your target is 60-~100 FPS it should do really well.

10400F has a little more life left in it; but there may be a title here or there that it struggles with. Given that you're fine with a 60FPS floor, you'll probably be OK for at least a short window. When you do finally encounter something that gives it trouble that's worth it, you should be fairly well-positioned on the GPU.

So I'd probably throw in the best GPU that you can. The 9700XT is a really great bet. The 9060XT 16GB/5060 Ti 16GB is more budget friendly.

Alternatively: sell the system used and do it now. In parts, it's probably a ~$600-700 build. You can 7600/9600 AM5 (CPU/Mobo/RAM) for ~$600 or 14600k DDR4 motherboard for $500 (slightly better results, but you're stuck without notable future upgrade paths unlike AM5). You'd still need a 850W PSU for $80, case for $50-$80, and 1TB NVMe for $150.