With an empty internal Cartridge Bay and the SD reader at 08H-0BH.
SymbOS behaves the very same playing an MP3 song from the G: unit.

this is the normal 4 colour SymbOS Nick mode

Now that you have some experience with memory expansions, you (with your friend) can try to make a simple SRAM/Flash ROM card for the expansion bus.
The first 74ALS138 is totally the same as the External Cartridge Bay.
Then you can directly connect 512K chips to the 138 outputs, like my first Flash   ROM card.
But there are already a lot of configurations and some expansions have put memory somewhere, so it will be good idea to add some configuration options:
Each memory chip needs its own 74HCT151:
Input A,B,C: A16,A17,A18
Enable (G): GND
Input D0-D7: any output from the LS138. Each Input represent a 64KB area of the memory chip, that will be enabled in the selected 512KB area.
The Y output of each LS151 will be the CE of every associated memory chip

For example D0:Y2, D1:Y0, D2-D7:Y2, then the ROM chip will be at: 40h-43h, 04h-07h, 48h-5Fh. The second 64KB is moved to cartridge area, then you can install there the quick memory test for EXOS 2.0/2.1 (on a machine with original ROM), when no cartridge is used.
Another example, SRAM at top:
On EP64: D0-D6:Y7, D7:Y6, SRAM at E0h-FBh, DCh-DFh
On EP128: D0-D5:Y7, D6-D7:Y6, SRAM at E0h-F7h, D8h-DFh

For a card I suggest to place a 7805 with a jumper to select or not +5V from bus Pin4A/B. Then the card could be also used with a soldered edge connector to plug directly to the machine. This will be a good alternative to the internal memory expansions, for who doesn't want to open the machine.