I use really love the
Ubiquiti Unifi APs for whole house and yard coverage. Rock solid, and very robust, high-end (enterprise) feature set. As they are APs only, you can use any router you like. I use a small, low cost but very rugged and reliable
UBNT router (with no built-in wifi) because it is also a POE switch to drive the APs.
One challenge with most modern consumer routers is you end up paying for wifi that is wasted, assuming you have different brand APs. A low cost, dedicated router solves this, as does pairing a router (with wifi) along with same-brand APs, such as the Synology suggestion. Synology feature set/interface is a big bonus too, if you need/want those features.
But one could use any router, including your existing Airport (with wifi turned off). for basic routing, DHCP, etc, with UBNT access points.
All that being said...based on your apartment size, I would expect one good, modern router should cover everything well, barring unusual interior construction (brick walls, metal lath, etc.). Still smart to plan for growth in case you decide you need more coverage (outside?), have a shadow or weak spot, or end up moving to a bigger space.