I think I found the solution myself.
This forum entry here helped me:
I changed the global contrasts to
options(contrasts = c(“contr.sum”, “contr.poly”))
before running brm. Then, later in the conditional_effects, to look at e.g. the effect of type1 alone, I used:
conditional_effects(mod1, effects = “type1”, re_formula = NULL,categorical=T,conditions = data.frame(type2 = NA, type3= NA))
Now, the reference is not always the same and makes sense!