What THA formicarium should they move into next?
I personally would get the mini hearth xxl once they fill up the mini hearth. You can just connect it modular to the mini hearth, if you don't mind them being connected. They will use both nests and while I have no experience with M. depelis, hopefully they are like the more common species and are very clean, thus they will not store trash in xxl(they will store it temporarily while processing), even if it's only half full.
THA recommended the Lair formicarium for them.
The lair is made for founding/small colonies.