Have you ever ridden a horse schooled primarily and entirely for dressage? It's very different than riding an event horse that's schooled in dressage.
Any dressage past second level, or even good, pure dressage AT second level, IMO and IME, requires a level of submission to the rider (and a frame!) that's counterproductive to good jumping.
You stated in your post that your horse has an "I got this" attitude in the jumping phases. That's cool, because that's exactly what you want in an event horse, or except at the highest levels, a jumper. Preserve that attitude in your horse, it's a good thing. Dressage training past second level completly subverts that attitude; it's supposed to.
I have two quibbles with "Jumping is dressage with speed bumps" and the current eventing situation. I say "quibble" because in principle, I agree with the OP - too many people start jumping without paying attention to the basics; and too many look for a quick fix rather than going back to the foundation. Flatwork does fix everything, I agree.
One, because of legitimate safety concerns and standardization of courses, pretty much any event at training level or below is decided primarily by the dressage score. Eliminaton of false ground lines and "bogey" fences and deciding at which level to add water, technical combinations, etc., were all excellent changes that had an unintended consequence - when pretty much everybody who shows up decently prepared can jump around the course, results are decided by dressage score. This means a horse that shows a frame, movement and submission appropriate for First/Second level scores above a true Training Level horse; and that then the other competitors (all of whom can jump the jumps as well) must chase that score. This leads to dressage specialists and jumping dilettantes, which is disasterous when you advance up the levels.
Two, there are far too many riders today who take "dressage is the foundation for jumping" to heart with too little education. They're the ones you see jumping with their horses' faces cranked to the vertical, with too long stirrups, little release and adding strides in every line. Please, before you protest, I don't confuse this with good, well rounded horsepeople who can adopt a variety of seats for a variety of situations. However, dressage being viewed as the cure for every ill creates its own set of problems in the undereducated.
Doesn't mean dressage is bad, it means a misunderstanding of dressage and/or classical principles is bad.