That said, my next router/gateway won't be from Ubiquiti. Though I'll keep using UI access points for now.
[0] https://help.ui.com/hc/en-us/articles/115001529267-UniFi-Man...
[1] https://help.ui.com/hc/en-us/articles/115012664088-UniFi-Int...
I'm still using Wi-Fi 5 because it's fast enough and cheaper. My central AP is a IAP-315, an IAP-305 in the garage, and another IAP-305 at the wall by the back yard. They're all PoE and linked with wired backbone to form a single big coverage area using a single elected IAP leader as controller for the rest.
You shouldn't have trouble buying grey-market ones as long as you are careful to stick to the same regulatory domain for all of them. Aruba gear is available as USA/FCC, Japan, Israel, and RW (Rest of World) versions. I have operated RW units in FCC territory (proooobably legally but probably not worth the risk) by setting them to "US Virgin Islands" so they match FCC-allowed frequencies and power limits, but linking more than one AP still requires the hardware to be same regulatory domain.
Check https://mikrotik.com/software for some demos and stuff.
For mad scientists though, the very open software stack is a good friend to have when 11th hour Requirements® dictate you must produce a rabbit without a hat, or rewrite your own domain-specific implementation to replace the Avahi service.
No experience with Mikrotic.
_On topic_: With cloud news like this, it's nice to know about the availability of Ubiquitis' Network Management System[1] which you can host and run wherever.
[1]: https://unms.com/