I’m still struggling like you, but it basically it works like this for me:
Make giant all encompassing navmesh in persistent level. Make a single navmesh for each tile. But you don’t need multiple navmeshes in each level, just single navmesh for each tile and it’s sublevels.
That tile navmesh is basically pointing where tiled mesh will be stored.
Recast actor has to be in persistent level with Fixed Tile Pool Size and Force Rebuild on load.
Then make persistent level invisible (IMPORTANT!) (eye icon) and load all your levels, press build paths and you might need to adjust Tile Pool Size and tile size in recast actor to fit your world.
It works fine for my test landscape of 16 small tiles. The reason I’m still struggling is because I don’t think it will be possible for 4000 tiles. Pretty sure I can’t load 4000 levels at at once.
So I’m trying to come up with the way to generate paths for each level separately and I don’t know yet how.