Thanks again for the link @dave14305
Successfully built and flashed an image with fw4+luci for Edgerouter X. Preserving the settings worked great, no modifications were needed whatsoever.
Stuff like Wireguard and SQM confirmed to work. The only change I had to do is changing usr/share/luci/menu.d/luci-app-firewall.json from /sbin/fw3 to /usr/sbin/fw3 for luci to work with fw4 as you had suggested in the PR.
If anyone here is interested, let me know and I can share the sysupgrade image.