It seems a lot of AirPort Extreme users have gone over to Synology, which is the route that I chose (Synology rt2600ac - $200). Synology used to offer a lower-powered version (rt1900ac) which was more within your price range, but it seems it was officially discontinued and replaced with the mr2200ac (~$140), a mesh router.
I've been using the Synology as my main router for a bit over a year now, and like it quite a bit. The blessing and curse is that it's capable of a lot more than the AirPort Extreme. It's a blessing because features like being able to form VPN connections to my home network and the newly added encryption of DNS queries are fantastic, and it's a curse because setting everything up takes a bit more doing. Time Machine backups through the AirPort were pretty seamless, but I had to make a special account on the router to get it working with the Synology, and initially it was still a little unstable (now works seamlessly); I still haven't been able to get USB printer sharing to work with my current printer (which is less than ten years old but getting there), whereas the AirPort handled it without any trouble. The router receives relatively frequent updates with additions of new features, and has many administrative and security-based features that are really nice.
Long story short, I'd replace this Synology rt2600ac with another Synology router when the time comes. I'll use my AirPort Extremes to extend the network if I move and have a need to, and if I need more devices, I'll use the Synology mr2200ac's. If Apple got back into the networking business and released an updated AirPort Extreme I'd be really tempted to switch back, but they're not giving any indication that they plan to reopen that division. Synology all the way for now, then.