Hi
Well done! Ingenious!
Your project downloaded OK but previewing it in VP (at least here on my old PC) was quite "bitty" I think that there was a lot for the program to handle with the number of tracks; clips and effects, nevertheless it seemed to export OK at your stated resolution. (1080 x 1920 9:16) Just played it through and apart from 2 small clips which were stills and one possibly misplaced clip that left a blank area it came out OK.
I think that some of the sluggishness noted in preview is due to the fact VP has to read the scaling, position and motion settings that each clip has on each of the tracks and still play it back at 30 fps. Here my PC was working full blast.🙂
It might be possible to do something like this in sections i.e. each using 3-4 clips at a time using the Split screen effect and then compiling the exported clips for a final result. It would also mean you could bring in clips to the vacant boxes at the required instant and use more of the screen. Scaling/image position in the boxes etc are set in a single effect.
e.g. using a 3 x 4 layout. You have 4 empty cells to either side here...

This might probably preview more smoothly.
All the same..a lot of work .Well done!
Nat