only ~900?

Not bad.

Typically see quotes in the 4-figure ranges.

You have to remember if you aren't DIY you've gotta pay for their labor, knowledge, and overhead.

Apartments are small and easy to cover.

Just get a more powerful router to start with. Maybe one with swappable antenna jacks.

Position the antenna perpendicular to the direction you want to aim them. Maybe replace one antenna with a flat panel directional antenna aimed where you need coverage fill.

If that doesn't work, you'll want to run ethernet (apartment, so tuck under baseboards) to the dead zone and install a wireless access point (WAP) at the end.