Are you really sure that a microcontroller with firmware is needed? From what I know:
- You can directly connect I3C and I2C target ICs to the same physical bus and use all of them at their respective maximum speed.
- A buffer/translator IC between I3C and I2C is only needed for connecting I2C target devices that use features for which I3C is not backwards-compatible, for example: clock stretching. The datasheet of the I2C chip in question should contain this information. I guess, such buffers/translators can be bought off the shelf.
- From the bus-side of things, the Jolla Phone only requires a small I2C memory chip at target address 0x50 inside the TOH shell which contains identification and initialisation information as specified here. It does not require complex stuff like a microcontroller or whatever.
- On that very same bus, you can then connect any I3C/I2C chips or ready-made modules of your preference.
EDIT: Here is how this basic identification circuit looks like in the case of the Snow White TOH: Jolla Phone 2026 The Other Half - How does it work? - #7 by moripeluka