How do I do that with the SSD cache? Everyone just talks about it, but I can’t find any documentation…

I have them at home and as such don‘t need to spend money on them.

Follow up question: Do I need a separate CPU cooler, or is the boxed Wraith enough? And how loud is it? I can’t really find info on that.

The NAS drives are here because they are from an old NAS which only uses a cheap dual-core Intel, has 2G of RAM, and boots from eMMC. So anything would be an upgrade…

I personally use a TPM with Measured Boot (so it doesn‘t give the key to external disks), and have a YubiKey and password as fallback options.

Well, that depends on the PC. Upgrading a laptop is quite hard (with exceptions). And with RAMageddon, a „real“ desktop may be prohibitively expensive. In Germany, where I am, a PS5 Digital Edition is 599,99€ (including VAT), while a good PC would probably be at least 1300€. But OP, if you can afford it, I‘d still recommend the PC for its versatility. You can do anything with that. Put SteamOS on it, its like a console. One button press, you‘re on a full desktop you can use for work, education, social media, etc. p.p.