Multicategorical x in bmlm mediation?

Hiya all, new to Stan here and I have a simple data-wrangling issue I’d like some advice on.

I’m aiming to run a mediation model from a simple repeated measures study, where x is the condition a given participant was in, m is a continuously measured mediator (measured only once), and y is a continuously measured DV measured at two time-points. Though it’s not necessarily tailored to 2-2-1 mediations, I was thinking I could still use bmlm–here’s a quick mock dataset of what the data would broadly look like:

And here’s the kind of basic model creation in bmlm I’m picturing at the moment

fit <- mlm(d = df,
           id = "id",
           x = "x", #??? 
           m = "m",
           y = "y", 
          iter = ...
          etc...)

The problem I’m seeing for use in bmlm is that x is multicategorical, not continuous, but I’m not sure how to specify that for bmlm purposes–any thoughts or ideas how to make a categorical x work in bmlm? Or is there perhaps an alternative way of going about this kind of model build?

replying for post visibility, any ideas on using categorical vars in bmlm? Or is there perhaps a better place for me to ask this kind of question?
Cheers!