mmh, on every river trip would have time for rolling, although I most times one of the shuttle drivers, but during the trip and at the end is enough time.
There are playspots like surfwaves or fast eddy lines, where breaking out and in, is exercised. Sometimes this ends up in a capsize anyway. There can be pools or wider parts of the river, where the water is deep enough and not moving fast, perfect for a roll. For our groups it is quite usual to asked another paddler whether he/she can come closer and keep an eye on you, during rolling attempts.
Or is an eddy at the bottom of a drop/rapid big enough, then we try it there.
This don't take long, only a few seconds. It will not hold up the group.
If it is the cold water you are afraid of? Though swimming is even colder and you get proper wet.
When you say your are in coaching, where is this coaching done, only on the river or also in a basin/canal? Why you can't asked someone from the club for a half hour session after the normal training or before? Or even on another day, special session for you or organize a wet session. A wet session not only about rolling, but rescues in general, rolling being only a part of it, mix it up with balance games. We have always great fun, if on the basin we usually take a variants of boat, there are seakayak, river runners, open boats. Exercising tows, throw lines, emptying boats, peer rescues, self rescues (getting back in the boat climbing, but also rolls)
That's why you have a club and peers, they are there to help you. It is not being selfish, when you help with the coaching and asking for some coaching for you.
Regards the roll and how it works, do you have really understand the roll? Some paddlers are just rolling and don't think about why it is working (especially kids). Others need to understand why and how a roll is working. You seem to be one of the later. As the penny dropped for you? Do you know why the head should come up last? Why you need to execute certain motions? Or are you just doing because you were told to do it this way?