As a rule of thumb i buy high-end to last me for ages.
I prefer Asus motherboards, the last one lasted 10 years and i only replaced it 'cos of bad diagnostics on my end.
I prefer AMD CPUs because, so far, AMD has stuck to socket upgradeability - that board came with an AM+ Athlon which i replaced with a Phenom. Current one’s Ryzen 5 and i can upgrade to Ruzen 9 if so desired. I’d say Threadripper is overkill, stay on Ryzen.
32 GiB RAM is nice, more is nicer, especially if you play with virtualization. You may want to remount your /tmp as an in RAM tmpfs (otherwise you won’t be able to compile firefox on gentoo).
I don’t care about graphics cards (beware some CPUs don’t support your mobo’s on-board graphics) but you mentioned LLMs… so… go gamer on that, can’t help you there.
Storage… really depends on your projects… 1TB HDD is more than enough for most stuff, find good brands within your budget/needs. I prefer SSDs because they’re quiet and curently have 3 Crucial MX500s on a ZRAID6 2 TB pool.
Same for fans: Noctua or be Quiet.
PSU’s probably… 400W? Dunno, always buy more than you need and you’re gonna need to feed that external GPU…