What I need to know boils down to:
What is the difference in installation procedure with treble compared to non-treble? (Installation from stock as starting point)
Does it need any special Twrp version? Where to get treble enabled twrp from if official twrp is not treble enabled? How to know if a twrp image is treble enabled?
Does it have something to do with A/B or A-only recovery stuff? (Haven't figured that out yet either)
Which treble ROM(s) are stable and fully working?