I have no idea what you want from us. We had or office and master BR in the attic conversion in Galgary where temps vary greatly and no AC and no reall issues opened the windows for ventilation and a fan. IMO a guest room should be comfortable for guests so I guess if I wouldn’t use it for my office becuse it was hot or cold I would not put my guest there either. I don’t really know why a slate roof would require it to be cold so no idea about that. I know tile roofs are very common in Europe where temps are often up and down .

I'm looking for folks who had planned to use finished attic rooms extensively and ended up liking it or, more importantly, not, due to heat fluctuations.

Thanks, but I don't think Calgary work would really be applicable, as the central Midwest often has weeks on end of summer temps in the 90s with high humidity.

I would not want to be in the attic with no AC. heat rises.

I have an older 1 story brick home with a walkup attic converted to living space about 20 years ago. These converted spaces are common in my area, in both 1 and 2 story houses. Pre Covid, when DH was on the road most weeks, I would often sleep in the main floor guest room.
Our biggest issue with the upstairs space is HVAC. The previous owner added a trunk line from the furnace to the attic but did not put in an air return. They added a gas fireplace which will heat up the space very nicely so a flip of the switch handles that side of the issue. AC is a major issue. The ceiling was finished so we do not know how well it is insulated but not enough. We did spray foam in the attic spaces under the gables but that isn't enough to stop major heat gain in the summer. Right now, we have a casement window AC unit which is sufficient to bring the temperature down to 78 or 79 F but it isn't as quiet as central air.

We knew that HVAC would be a problem. We considered extending supply and return lines but that would have been costly and taken a good portion of the main floor closet space. A minisplit system was a better option but ultimately, we decided to wait and see how much it was needed before throwing several thousand dollars at the issue. Before the upstairs floor and basement ceiling were finished, we put in an empty run of conduit from the install location to the electrical panel for a possible future unit.
Perhaps a third floor attic would be different than a second floor space, but I don't find it an issue to run upstairs for this or that. It isn't really any different than going down to the basement for my sewing space. If you have mobility issues or need to answer the door often, the answer might be different. For me, the physical separation of the space is a positive aspect as I can focus better without the main floor distractions like fridge, snacks and tea kettle.

We just recently converted our 3rd floor unfinished attic in a 100 yr old farmhouse into a game room/playroom for our kids. We live in the midwest, so the attic was always very cold in winter and boiling in summer without any HVAC. We installed a mini-split heat pump. It's great to have a conditioned space that has its own temperature control. Originally I wanted it to be a bedroom, but I had concerns about fire escape with only one window--we did end up adding a 2nd dormer, so it would really be ok as a bedroom now. Is there more than one exit in case of fire? That would be my concern as a bedroom.