Without significantly complicating can do this:
00-03: onboard ROM
10-13,20-23,30-33,40-43,50-53,60-63,70-73: Flash ROM (standard ROM extensions segments)
80-83 (jumper switched on 04-07): Flash ROM
80-83: RAM as ROM when Flash switched to cartridge segment
90-93,A0-A3,B0-B3,C0-C3,D0-D3,E0-E3,F0-F3: RAM as ROM (standard ROM extensions segments)
9C-9F,AC-AF,BC-BF,CC-CF,DC-DF,EC-EF: unused
F8-FB: internal RAM (jumper mapped on external RAM)
FC-FF: onboard RAM
others not listed above: RAM (2.5MB - 64KB at F8-FB)
It seems strange, but such an arrangement does not require a complicated method of addressing.
Simply connect the address lines A18-A20 of Enterprise to address lines A16-A18 of the memories, and decode chip addresses with the A16, A17 and A21.
Wasted only 64 KB of RAM, if internal memory expansion is installed.
What do You think about it ?