That is a good policy, no good upgrading to something that is not wifi6 I guess.
Ubiquiti being slow to upgrade possibly means that they are more likely to get it right 
But anyway this is about modems and routers, not access points.
From what I read, the value if WiFi 6 for a HA environment full of ESP, Tasmato, Shelly, Google current, Apple connect, Amazon is way far off and will require a lot of upgrade cost. I would focus on VERY solid current (pre WiFi 6) support and the ability to get very current information from these devices and WiFi hubs. I have been exploring both Mikrotik and Ubiquiti for these requirement for my HA environment. So far, as a total network noob that has spend about USD 1k on both, I give the nod to Mikrotik. Both seem solid, but I have been able to programmatically extract information from Mikrotik with greater ease with Python. Don’t get me wrong, both will require a considerable commitment to get running and data extract. The cost point for the Mikrotik devices also seems lower then for Ubiquiti (if you remove the ‘advance’ feature’ that Ubiquiti offers but I am unlikely to use), this is if you strip off many of the higher end functions. Sudo as POE and 5 ghz. If you want solid 2.4 Ghz WDS hubs for Iot devices, I am leaning to Mikrotik…
I am benchmarking the following:
Mikrotik RB4011iGS
Mikrotik HaP AC2
vs.
Ubiquiti UniFi Dream Machine
not totally equal, but I think both need a second WDS wifi hub in almost any USA single family home
both moving from 1st gen Google Mesh Wifi (2 units) (which was solid, but totally closed to reporting)
The main advantage of WiFi 6 is in the OFDMA, which is NOT backwards compatible. If you are using a mix of non 6 hardware on a radio, it will basically be an 802.11ac wave 2 access point.
On the other hand, if all your 5Ghz stuff is 6, the 2.4GHz radio having slow old iot stuff on it will not drop down the 5GHz OFDMA to OFDM.
So, if you are going to use older hardware on a frequency, 6 will not give a large advantage, unless it has an 8x8 radio antenna, as most AC APs are 4x4, and you are going to have more than 4 high speed clients connecting simultaneously
I second the use of OPNsense. Unlike many routers which have a limit on the number of connections, OPNsense has no such limit, as long as the hardware is up to scratch. If not, upgrade the hardware. Very easy to set up and has an anti-virus plugin.
A couple of commercial grade APs will give wifi you won’t experience with a normal router.
I’m solidly in the software edge router camp. In my case, I use pfSense which has a common lineage with OPNsense. To work within a budget, I used a combination of heterogeneous products which includes a D-Link managed switch and TP-Link wireless access points. To be honest, it took some amount of time to learn how to configure everything, but I got a basic setup operational in a day.
I would stay within the same product line for access points to ensure they play well together. The more capable ones can facilitate seamless roaming and beam steering between multiple APs, but they need a controller. I used my desktop Windows machine to host the controller, but it can also run on a Linux box. Alternately, the manufacturer typically sells a single-purpose box, or provides a cloud service for the controller.
There are trade-offs involved with any decision. I’ve gone to this source for making some of my purchase decisions and configuration tutorials: