Just picked up a HAP Mini aka RB931-2nD r2 and I can successfully flash 19.07.10 but I was wondering if its worth attempting to upgrade the to ath79, and it seems that the switch isn't supported correctly as it can't see eth1, and all 3 ethernet ports are appearing as eth0 switch.
You will probably need to use swconfig to see a hardware switch: https://openwrt.org/docs/guide-user/network/vlan/switch. Ath79 will not change this—it does not use DSA switch drivers for any QCA/Atheros hardware switches.