Finally got my hands on an FP300 at Amazon, just by checking often. Blinks available, then in a short time, unavailable. I caught a window.
Setup was a breeze, added to Z2M Zigbee running in Docker linked to HAOS via MQTT.
Not even familiar with the Aqara app but fudged my way through.
Opened app, something about cards, chose default 
, selected the found FP300, chose Zigbee, it downloaded latest firmware no issues.
Backed out of app, don’t have or need a hub other than HAOS.
Added to Z2M, not found, clicked button got a couple of blinks, added, done.
After reading online, I was a little apprehensive, but all went well fairly quicky.
Kept default settings for now mostly, changed a few delay times, have to find a permanent mounting spot now. Will monitor the battery. 