FF6Tools - Browser-Based ROM Editor

Also, is there a way to add more treasures? if I look at the chest data doc from Imzogelmo, it seems like there's room at the very end in map 415 (hidden map) but I suppose those requires space in SRAM and they all have the same Encoding byte:bit except the last 3 which uses different ones.

The SRAM right after the treasure chest bits is free, you can add more with the regular FF6LE or FF6LE Rogue. Maybe FF6Tools does not allow that SRAM to be used for treasure chests..

I know Madsiur did some trigger expansion with FF6LE and ZoneDoctor map editors but that's about it (download link is dead btw).

Which link are you talking about? The link for FF6LE CE on the utility page is working..