I was looking to do something similar and ended up here (I guess your “as many keywords as possible” thing worked). Here’s what I came up with that worked for me. (geometry and tile adjusted for your needs)
montage -border 0 -geometry 660x -tile 3x3 tile* final.jpg
The files get added to the tiles horizontally, so, for -tile 4x2, the disposition would be:
1 2 3 4
5 6 7 8
The numbers being the relative positions of the filenames in the argument list.
As far as I can tell, tile* will expand alphabetically, so you should either specify your filenames manually, or rename then so that they’ll sort appropriately, e.g.:
# top row
tile_r0_c0.jpg
tile_r0_c1.jpg
tile_r0_c2.jpg
# middle row
tile_r1_c0.jpg
tile_r1_c1.jpg
tile_r1_c2.jpg
# bottom row
tile_r2_c0.jpg
tile_r2_c1.jpg
tile_r2_c2.jpg