Hello,
Sorry for this late reply. I managed to install OpenWRT after dowgrading RouterOS. It was quite painful and complicated. In the end I got bad performance under OpenWRT, especially on Ethernet, so I dropped the idea of using OpenWRT on these devices. I have 5 of them and will resell them.
On converse I confirm WPA3 support, which is a nice feature.
Please follow this link:
https://forum.openwrt.org/t/mikrotik-hap-ac-tftp-kernel-image-booting-fixed-sfp-fixed-poe-in-fixed-poe-out-fixed-slow-switch-not-fixed
Kind regards,