OpenWRT supports that.
I keep hoping that one of the OpenWRT snapshots will fix it, but this is something I've been fighting with for years on multiple pieces of hardware.
Separate 5G and 2.4G SSIDs are no issue in my experience-- it's multiple SSIDs on one phy where Atheros goes wrong.
I really want WiFi6 gear... but given that there's no real OpenWRT support yet, and how long it's taken 802.11ac to mature in OpenWRT (it arguably hasn't yet), it's kind of discouraging.
maybe you have a known-bad hardware in regards to driver support, but that is absolutely not to be expected.