weechat-matrix is really not a dead project. the maintainer works for Element, and is working on matrix-rust-sdk (and thus weechat-matrix-rs), but weechat-matrix is alive in well and in no way unmaintained or dead.
Okay, but I'm not going to go through the effort of setting up weechat-matrix if I know I'm going to need to later throw all that effort away and switch to weechat-matrix-rs. I'll just wait for the latter. It is, for my purposes, dead.