Thanks a lot! It might take me some time to figure how to use it in my case, but that does appear to be the right way to go. I’ll post what I did if I succeed so that others can see it. With all the recent emphasis on latent mean centering, I think many people might be interested in how to do this in R.
BTW, there is a related post here Modeling latent means in brms for multilevel group mean centering