This one's a bit controversial (which is to say that no matter what I say someone will disagree with me, possibly vehemently), but I don't think you always have to do rest-strokes with tremolo. I'll often start a phrase with a rest-stroke and then use free-stroke for the rest of the phrase, and sometimes I'll use all of one or all of another. If it's just not happening for you with the rest-stroke that's no reason to abandon tremolo all together.
Having said that, almost everything in the realm of technique is about giving yourself time to prepare for the next movement. If I said "play a rest-stroke with your thumb, wait 30 seconds, then play i,a,m,i as slowly as you like" you probably would have no trouble at all. So it comes down to the transition from playing the thumb rest-stroke and then playing the i,a,m,i. The practice would come down to working on that transition so that it's smoother and takes less time until eventually it feels natural and doesn't require a pause. You might have to rejigger your right hand/arm after the rest-stroke (and you may find that it's just not worth it) but if you give yourself the time you might also discover that there's a way to do it.
But again, if it feels terrible don't sweat it. You can still have a great tremolo!