> I don't understand why Anna's Archive has such a convoluted donation system.

What? It's one page with a bunch of very clear options.

> At first glance it looks like it's trying to push a subscription on you...

On the one hand, fair. On the other hand, this is prominently displayed on the donation page:

  Be aware that while the memberships on this page are “per month”, they are one-time donations (non-recurring). See the Donation FAQ.

  Do memberships automatically renew?
  Memberships do not automatically renew. You can join for as long or short as you want.
  

> I found no convenient single-link crypto donate button where I could just send some money whenever I want.

From their Donation FAQ:

  Can I make a donation without becoming a member?
  Sure thing. We accept donations of any amount on this Monero (XMR) address: 88gS7a8aHj5EYhCfYnkhEmYXX3MtR35r3YhWdWXwGLyS4fkXYjkupcif6RY5oj9xkNR8VVmoRXh1kQKQrZBRRc8PHLWMgUR.

Yeah, what OP is bizarrely describing as "push a subscription" with a (completely false) "im 12 and this is deep" insight about Anna Archive becoming the very thing they swore to destroy, can accurately be explained as them trying to ensure you get the benefits entitled by your donation level via your access key (linked to zero personal information).

I donate in 3 or 6 months chunks typically, if I forget, I don't get hassled to resubscribe or anything (nor could they even contact me for any reason), I just lose those bonus download benefits until I do it again. I could also generate a new key each time but it's convenient keeping the same one in my password manager so I like the way they do it now, basically works just like LWN.net.