The honest version from someone who did leave a comfortable job for a more interesting one.
What I gained: the work was genuinely harder and I learned more in six months than in the previous two years. That part was real and I would do it again.
What I gave up, and underestimated: knowing everybody, knowing how things worked, being the person people asked. All of that resets to zero and it takes the better part of a year to rebuild. For the first three months I was slower and less useful than I had been in a decade, which was uncomfortable in a way I had not anticipated.
What I did not get: the feeling of never being bored again. New work is interesting for a while and then becomes work. If you are hoping the boredom is permanently solved by a change of employer, it is not - you are buying a couple of good years and then the same question returns.
So my honest advice: try the internal route above first, because it gets you most of the benefit for none of the reset. But if it does not work, do go. The version of this that ends badly is not leaving too soon; it is staying five years because it was fine, and finding out what that cost when you eventually have to move anyway.