I have found it useful to train IS up a bit then Rebound and while doing that you will get some ID pops.
My 20 year old is on the 21 NT side a year early.
IF you do alternate which is the safest or focus on 1 then the other best to do it in this order IS then RB > ID. Doing each until it pops then go to next and repeat is a good way to do it. But I got on a role and did IS until 10 then hit the others.
But doing rotation means your guy becomes more useful sooner.