@globe79, well I just went ahead and updated my TWRP to 3.2.3 (couldn't find the zip for 22.214.171.124) and flashed aosp-9-20190422-huashan-adriandc.zip along with Open Gapps arm-9.0-micro-20190601.
There were a couple reboots and little bit of lag initially, but now (couple of hours later) it seems fine. Sort of forgot about doing a factory reset, and didn't realise how much would be preserved from the previous version, even with the new Gapps in place.
Overall, the ROM seems at least as smooth as before. I actually had some lag on Oreo, but less(?) with this one (this time, I started by flashing the boot.img, does that make a difference?). Not an expert with these things really... :silly:
Obviously not the fastest in 2019, but still seems like a great ROM. Will be testing it more now.
Thanks once again @AdrianDC, much appreciated.