Treble on OTA update is a no no no. The risk is too great and no OEM will be willing to do it properly via OTA... especially if you already have A/B partitions. I personally don't want it. I'm not willing to risk my whole device on a simple update. A Treble update will not only destroy all data on memory but it could brig your whole device and in which case you'll need to manually partition your memory on low level (through qualcomm).
8.1 is currently in Beta tests and P is also on beta... why would you want a beta of P when you say 8 is buggy enough. You honestly believe that a beta version will be bugless??? It's the whole reason of alpha and beta tests is to find and fix bugs.