You got the right idea. Remove that smt device you point to in the pic. That disconnects the internal antenna.
I would use the newely vacated pad (the pad connected to the thru hole) for the center conductor of the coax.
Scrape back the green solder mask adjacent to it (between the pad & the round exposed copper just above it in the photo) for the coax shield.
You want _very_ short & direct coax-to-pcb connection. See for a pic of two oem coax-pcb terminations. Its a very early WAP54G.
Where you show the coax connected would work, but shorter is better.
Don't forget a strain relief on the coax :)