Hi @fstudart,
first-distances is really only going to make sense if you have > 2 states. It will work with 2 states, and the results can still be used, e.g., to plot how much a sample changed between state 1 and state 0. But it will not be useful for linear-mixed-effects.
Correct — those methods actually effectively calculate the first differences/distances between two fixed states but then provide some statistical testing and plotting. Those will be much more useful for your purposes.
Looks like you opened up a separate question about first-distances (thanks for using separate topics!) — let's move the conversation will move to this topic: