So, just discovering the joys of virtual monitors available in the kernel now. NAS now has and gives me sunshine. Bazzite lets you put your own monitors' (ujust setup-sunshine - Configure Virtual Monitor) edid into initramfs so sunshine (and KDE remote access if you want laggy but cut and paste enabled) work without a monitor connection (or dongle), very cool. Just thought I should spread the news and this is vaguely associated.
Not really relevant to the X11 question, except that it provides a (superior) workaround for some issues..
To the actual question, xtool for automating UI stuff hasn't been completely replicated, but given it's an insecure horrible solution I'm OK. Also running a program through a remote connection on a beefier machine was useful at times, is there an equivalent other than remote access?