@Wintermute_BBS https://www.msx.org/wiki/Category:Flash_Card_Interfaces has a nice overview of flash card interfaces for the #MSX you might find useful. :)
For its price and current availability, https://www.msx.org/news/hardware/en/msx-pico-cartridge has piqued my interest, though ideally I'd probably want a #GR8NET cart for its additional functionalities.