Honestly, the only dynamic thing any part of life has, on an exclusively individual level, is the learning aspect. It's new and foreign to us, even if the subject matter may not be to someone else, so we have to basically start as a clean slate. It's also temporary, as it'll no longer be surprising once you've learned it. Proper execution is generally an act of repetition, which comes AFTER you learn how to do it. There's nothing actually challenging about mindlessly throwing something at a wall repeatedly or doing something you've done countless times in the past. Is the Rubik's Cube difficult to someone who has solved the puzzle hundreds, if not thousands, of times? Is basic addition/subtraction difficult to an accountant?

The idea of adding RNG to keep a fight fresh would not work well, if that were to be a retort. In the case of boss fights in a game, adding every element of RNG to the encounter makes it difficult to balance. If you wanted to add an ability to the boss that makes them invincible for 10sec while you have to kill/lure adds across the room to key positions, it'd be possible for this boss to do this repeatedly because RNG, thus possibly making it impossible to win. If you limit how often it can be used or at what intervals, it becomes predictable... no challenge once you've seen it and "learned" how to get through it, despite the intent of having that RNG there for it. In other words, you'd be pretty much where things are now. Boss fights like Bismarck EX are very faceroll mechanically. It's more or less all about your setup (gear, jobs, etc) and familiarity of the story version. Fights like T9 are relatively refreshing, albeit possibly TOO lengthy in the mechanics lol. T9 could have easily done away with something like the hot/cold mechanic and still have been new and challenging while you progressed.

Learning is a huge part about what makes a game fun, despite the negative connotation a rather large percentage of the population might view "learning" to be. It's not scary, and is in fact, something you do daily... hopefully, unless you're a vegetable or dead. It can certainly be overwhelming at times, but it rarely hits that point for a game like this. T9 is one of the only examples of that, where learning mechanics can be overwhelming (not the part of actually being able to do them). You're going to fail as you learn, that's just normal. So unless you want to do the exact same thing for every single encounter time and time again, you're going to have to suck it up and learn something new once in a while.