Though you'll probably end up with Atheros wifi chipset on modern hardware... and I've found the OpenWRT drivers to be extremely unreliable when providing multiple SSIDs--- crashing every few days instead of weeks of uptime.
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.
I bought a Linksys WRT3200ACM specifically for use with OpenWRT after a bunch of research. It's modern hardware and based on Marvell, not Atheros. I don't have lots of SSIDs, but I do have separate 5G and 2.4G SSIDs, and they're working well enough.
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.
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.