802.11ax devices that can at least support multi-ssid and vlan tagging?

I've split my router out from my Archer C7 to a powerful SBC running openwrt. I'd like to increase my wifi throughput so I need something that can create multiple SSIDs and tag them for my VLANs (assuming I will need OpenWRT for this, but not necessarily?). I'm looking at 802.11ax devices but none support OpenWRT as of yet. As long as I'm using the 802.11ax device simply as an AP, are there any stock 802.11ax firmwares that will allow me to tag vlans by ssid so they can be routed properly by my openwrt box?

Even your C7 will allow you to set up multiple SSID's and tag them to different VLAN's, so I do not really understand the question? That is rather basic OpenWRT functionality.

As for ax devices, as long as you can get OpenWRT working, any will do. Linksys E8450 is already supported.

I need more throughput.

As I mentioned, Linksys E8450 is supporting .ax. That being said, I have a fleet of Archer C7 v2's running as AP's and they are reliably doing 500Mbit/s in iperf3 using 5GHz and 80MHz channels.

If you splash out and buy a complete new fleet of .ax routers, and get them working and have .ax capable devices, you might not get significantly more bandwidth with .ax. 800-900Mbit in best case.