Not pretty but works. Compared side by side with another identical XIAO 52840 BLE, I got @83dB vs @92dB about 30 feet away inside another room (one wood wall).
Trimmed a spare external 2.4GHz antenna (careful with traces) and used the outside coax cable only. Instead of an actual antenna, a single solid-wire should work probably just as good as long as you have a straight unshielded section properly sized at the fraction of the 2.4GHZ wave length.
Connected antenna to the unused row of microphone solder pegs (left side row when looking from the existing chip-antenna side of the board). This row is connected to the antenna signal internally. In the photo it looks like it is soldered to the other row but just because I could not get a smaller solder blob.