My janky(-ish) but fun plan is to use laptop motherboards, either from broken laptops or AliExpress. Wire them up with an ATX powersupply and that should be a cheap and powerful cluster. If you want something less hacky the same idea but with used mini-PCs from offices.

Well, if you use DDNS I assume you have some kind of server behind that. I just self host a godns container. No need for any service except an DNS API. I use cloudflare. But my IP only changes rarely, so I can’t tell you how fast that setup propegates