For what its worth - I got started in UniFi when my Asus' wifi got flaky. I turned off the radio and added a UniFi AP (AC-Lite). Problem solved. I just had the AP sitting on top of a bookcase, not even noticeable. It covered my then house at 1700 sq ft. Maybe just adding an AP could improve your situation for now. (In a different house now, I have a complete UniFi setup - including that AC-Lite, which is doing fine!)
You could take a look at the UniFi Dream Router (wifi 6 4x4 MiMo AP built in) or Dream Machine (wifi 5 4x4 MiMo built in) - both are all-in-ones.
Generally, external antennas don't do a great job as add-ons. At the frequencies used, the signal loss in cables can be significant, especially the received signal (vs. the transmitted one). Also, the antennas on the box work in conjunction with each other (things like diversity reception). The devices are engineered for the antennas provided, and you can't just peel off one of four without affecting the function of the wifi transceiver.
u/Somhlth has the right idea here - put the actual AP out there somewhere (protect from weather), or add an actual outdoor AP like from TP-Link.