What logic is that? Just because the rest of the network gear is older, the router should be a few generations behind too?
The reasons to stick to Asus (ongoing security, recent code (again security) used in core firmware, the longest support in the pro-con sumer industry and RMerlin's firmware and the forks thereof) far outweigh any cost savings over the 3 to 5 years most people keep and use a router for.
Airports aren't that far behind - the AC Extreme is AC1900 class, and a decent performer - it's relatively stable, just received a firmware update a little over a month ago, and it just works for the most part...
From a WiFi perspective - it's the same WiFi chips as the other popular broadcom based Router/AP's like the RT68U and the R7000, and it has more horsepower under the "hood" as it is using an upgraded version of the SoC used on other Broadcom AC1900's...
It does have a stateful firewall (based on the NetBSD pf firewall, same as pfSense), which many overlook, and the Guest Network functionality can be extended to multiple Airports, unlike other vendors - it has a pretty good QoS implementation out of the box, sadly no access to tweak it, but it really doesn't need tweaking.
From a security perspective - not having an embedded webserver (or SSH/Telnet for that matter) is probably a plus considering the recent DDOS attacks that are leveraging weaknesses in Asus, Netgear, and DLink devices...
Outside of the Airport - the Netgear R7000 is also a good choice for an Apple centric network - it pretty much works, but like the Airport, there are some IPv6 issues with some operators (which Asus also has problems with).
For an Apple centric network like OP has - I cannot advise going Asus as AsusWRT (and the forks) tends to be very problematic with certain Apple vertical features like AirPlay and Wireless Wake on LAN (which depend on Multicast DNS for discovery). It's not the Asus is bad, not hardly, but this issues have been outstanding for a long time, and there's little indication that Asus really gives a darn about it, as it continues to be a problem for Apple centric networks.
Going back to OP - the current Router/AP is a Buffalo Airstation (which there are many) - and their Broadcom AC1900 class devices are built on on DDWRT, and are fully supported by DDWRT as a third party development, so moving to a "new" device might not be productive in any case... maybe it's just a firmware upgrade to the full DDWRT platform.. what I like about the Buffalo devices - their very similar to Asus, except much better quality on the HW side... more attention to detail, better quality components...