Typst, a new markup-based typesetting system, is now open source
There's also Sile: https://sile-typesetter.org/LaTeX (Well, LuaTeX) powers my E-ink newspaper (https://imgur.com/a/NoTr8XX), but I've been curious to use it as a vehicle to try out some of these alternatives and see if they can put together a non-trivial layout.