Again, I am using sprite objects acting as tiles (animations as tilesets, frames as tiles) as Ashley & Rexrainbow suggested. This was fine at first, but now that we have many tilesets there's A LOT to load, causing preview times to be upwards of 20 seconds.
My solution was to no longer use sprite objects acting as tiles and instead use the tilemap object (which didn't exist at the time we built most of our levels) but it's impossible to get that working with TMX Importer due to the tilemap object's limitations (one tilemap per tilemap object, etc.).
So, again, if we could just have the tilemap object support multiple tilemaps, and the ability to place any of those tiles on a single canvas, problem solved! I'm not really sure how Ashley expected 1 tilemap per object to be enough in the first place!