Thinking back not many years…
I think there are 2 potential problems with RT, firstly nervousness about using it and 2ndly knowing what to say. In my case the first was not an issue (ex military) but the 2nd definitely was with occasional “brain death” however much it was rehearsed. I know of at least one other person who was confident using RT on the day job but not so flying.
I would therefore suggest that RT should come very early for the easy things where you can pre-brief and/or where it’s pretty standard stuff such as a call for taxi or in the circuit, but leave more complicated things (cold calling an ATCU) till later, possibly much later depending on the student.
Over