If the neighbors are close enough together (5GHz range) and are you are setting up a mesh network for them, would something like the Linksys MR8300 be interesting? It has a second 5 GHz radio for mesh backhaul and very good WiFi range. The MR8300 is ipq4019 based - quad core ARM and an increase in performance from dual core MIPS MT7621.
Elsewhere on the forum, another user reported buying a like new refurbished MR8300 for 44 Euro. As of yesterday, they are $40 on amazon in the U.S. If you can find it, this would be at the top end of your price range, so perhaps not worth the cost, or perhaps it could be a deluxe option now within reach due to current pricing?
We are in our way to get rid of Ubiquiti NanoBeam/PowerBeam hardware and about to deploy fiber instead. It has worked good for us, but in the last couple of years things are getting worse. Interference is much greater now than it was in 2012 and speed is now less than it was then. Furthermore, there is now some radar somewhere making our main AP to drop connectivity. This causes avalanches of Telegram messages from our "strictly users" to our "geek team". Being a not-for-profit, we need reliability to maintain our geeks sane, well, at least as sane as possible.
So fiber it will be. Substituting a fiber backbone for a wifi one will surely improve reliability, and users will also get speedier connectivity. We think the improvement will be quite noticeable even with those old routers, that's why we aren't in a hurry to get 802.11 ac/ax routers. But, security-wise, to have 18.06 or even older on these devices isn't that good, so we need 16 MB flash to put up-to-date OpenWRT into it.
Depending upon the functionalities used, there are a lot of options to reduce RAM or flash requirements. Will need custom openwrt image(s), though. 8MB flash should suffice even for some future. Not enough when buying new routers, of course.
Even possible to squeeze a more or less recent openwrt into 4MB.