My bad, I really should’ve searched - thanks.

I’d recommend using a static site generator e.g. Hugo, Zola They use a template engine to create the pages, so you can write all your pages in markdown and these get built into slots in your templates. This gives you the freedom to write a very lightweight template such that each page is your template header plus your converted markdown. I’ve not touched Hugo in a while, but writing a Zola theme is quite straightforward and minimal.

Mirage supports multiple accounts. It’s still in alpha but worth keeping an eye on. Most features work (including E2EE), but a couple are missing (emoji reactions aren’t shown/usable, and if someone edits a message it is instead shown as a new message).
Fractal currently does not support E2EE or multiple accounts, but E2EE is coming in their new branch (Fractal-next) and multiple accounts are on their to-do.