The main reason I advocate for not jumping in both feet and entering a full trial from the get go is it's a much different environment from practise and it's a very long day. I don't expect my dog to be used to that from the get go and I want to gradually work them up to it and not just expect them to be okay with a super long and taxing day. I want to set my dog up to be successful and not overwhelm them.
And for the record i'd do that with ANY breed i had, not just Izzie.
It could be because of all the other stuff I do with my dogs but they don't find trials or shows to be all that stressful. Of course, they are socialized at them as young puppies having to hang out at them all day and we go to conformation shows, overnight trips, more conformation shows, more agility trials, multiple training venues, adventures in strange places, etc. Going to dog shows/trials is part of life from the very start.
Whimsy was trained almost entirely in my backyard and is extremely reliable at trials and has been since her first trial. I took her to one workshop, a few run-thrus at the two local training places and one outdoor run thru then starting trialing her. I just don't see trials as that big of a deal for most well socialized, well trained dogs providing they enjoy the game and have been trained well enough. I'm not sure Whim has ever even noticed people in the ring or anything going on outside of the ring at a trial, which many people would say would be a drawback of backyard only training. She knows her job is to run and she is very focused on that task
IME Many cases of trial stress seem to stem from training issues or not being fit enough for the task. People around here always seem so eager to start trialing without thinking about the fact that they can't reliably keep their dog's attention for short sequences in training or the dog isn't confident on all equipment or doesn't jump real well. They enter their dogs and hope they don't make too many mistakes to Q. They repeated allow the dog to run around the ring, visit judges and ring crew and do everything but focus on running with them. Those dogs, I agree shouldn't be entered in multiple classes. Those things quickly turn into hard to break habits. I'd argue they shouldn't be entered at all until they're ready though.
Of course, some dogs have different individual needs that make it more ideal to get into trialing slowly and carefully. I know a few seasoned agility dogs who still struggle with more than a couple runs day. And some people too LOL
In this case, I don't imagine Adrianne is entering a dog who she doesn't feel is ready and Sloan seems like a well socialized dog that does lots of stuff. I don't think I'd worry that she will be overwhelmed by the trialing environment but I don't know her and could be wrong. But I think this is Adrianne's first ever agility trial? If so, I think she should enter the class or classes that she feels most comfortable running. The first trials can be quite stressful for humans