I have an unusal setup. I have a UBNT
EdgeRouter X SFP. Small, robust, steel case, fanless (silent) rugged, and no wifi or bells and whistles. Not consumer-friendly...alot to wade through/not use except in enterprise or WISP management.
I don't really recommend an Edge Router to consumers, simply because the interface and configuration is not meant for a non-tech person (consumer) to manage.
But it is small, low cost, reliable, and does POE for APs. Just a POE switch can cost nearly as much...so it is a great value and makes for clean cabling (with up to 4 APs or other wired devices).
I wanted to avoid different brand APs too, and knowing I was going Unifi made it easy to choose a small, non-wifi router. Not that many choices out there, especially with built-in POE. It just works...no reboots for many months, typically only for firmware updates, not for any problems.
You could try running Unifi APs with the Synology, but I would be inclined to turn off the Synology wifi to avoid any hand-off issues...and to keep it simple. It may seem like a waste, but the Synology has great features outside of wireless, and is easy to manage, so most of its value is in software IMHO. I have never tried to integrate Unifi APs with other brands of APs...I see
this discussion of how to do it, and you can see some of the possible problems or shortcomings.
Unifi makes a s
mall wired router, and it uses the same interface/system as the APs, which is tempting. But it costs more, would require a switch (more cost, more mess), and it doesn't get great reviews...folks have said it is underpowered. So, at least for me, the Edge router was: smaller, simpler cabling, no performance issues, and much cheaper. The only downside is a separate tool/interface to set up and manage.