For the external intervalometer tests what microSD cards did you tried? Maybe faster card will allow shorter intervals to set: maybe 2,4 secs or less… I use a Samsung 128GB Evo Plus. I will try different intervals with this card in the near future.
I already tried the emild Reach and worked mostly fine. Based on my test in urban area (not in urban canyons) but between the 10 floor block of flats I had +/- 25 cm precision with fix corrections and +/- 75cm with float corrections. The test was made walking. Sometimes Emlid Reach establishes the float or fix only after a long time of waiting (1-3 minutes), but once it happened that I could not get the position (not even single) for more than 15-20 minutes - being in an airplane. Otherwise I used a lot the Reach for aerial imagery collection.
When you make the stitching in App you will find some strange duplicates on the stitching line. Stitching it with PTGUI (I used a template with more than 50 control points) I had a strange stitching error. See in the attached centimeter graded image. The distance between the camera and the centimetric image was around 70cm, but the reference image was held more in a square shape not in a circle shape. See the stitching problem on the attached image (I also put there the InApp stitched image - the result looks quite the same):
This stitching issue may damage the result and the accuracy of the feature extraction from the Xiaomi panoramas.