Just a quick update:
I'm on ax3600 using firmware SNAPSHOT - r23845-abc536f547 . This one works very stable for me. It contains fw_build_id WLAN.HK.2.9.0.1-01862-QCAHKSWPL_SILICONZ-1 .
But when I upgrade to a recent snapshot containing a newer version of the wifi firmware since https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=eb8ddf5d5c9df9e3ca97c12ee2e7685ffc4c88be;hp=a9cc3708e0c3c4869711a9ba4b9c1437ed250721 , both of my devices fail with out of memory killing processes in a few hours (4 to 8 h, probably depending on the network load). In the web ui, I can see my ram decreasing. Where before there was always more than 120 mb ram free, it drops constantly and about ~ 23 mb free the oom killer kicks in making the device unusable until reboot.