Condensed answer to complicated subject would be the topic here. Shrinking discs do work harden the metal. Bottom line here is that this is a high skilled job for anyone & a bad start to it makes it that much harder of a process to get close to perfect now that you added in some work hardening.
Sad to say, but Chris could be so right about it being easier to remove the whole roof skin to work on OR even replace. I learned the hard way to remove a long ass el camino rear quarter panel to work on rather than trying to repair a difficult area in place. R &R Cuts & welds are done near seams & edges where metal is less prone to warpage.
I didn't like hearing it either, but sometimes you have to do more work to do less work with better overall results.
Of course your next plan of attack depends on your individual results & goals.
As with any metal repair job before filler, high spots are far worse than low spots as they result in more filler everywhere for a long ways out to correct.
Also, think it's safe to say even the best metal men wish they were better, only 1 way to learn..... Some think I've done awesome on projects. They either didn't see how it bad my work looked early on or can't figure out how I got it straight in the end.
You can see some of my widebody project metalwork on my flicker link. 2 1/2" flare out at center along with using 69 charger upper quarter sections for new upper shape. Held on & off of vehicle with screws till it's minimal filler worthy. Main shaping in a custom reshape comes first, which can be pretty f-word ugly. Mine is currently what you call close to English wheel time. Hoping for bragging rights smooth, but a little heavier than skimcoat filler in spots won't be the end of the world.
Sometimes with bad dips, you may be better off hitting them slightly high in spots with a heavy plastic teardrop hammer to get closer to needed shape quicker, then working it down a little less dramatically with hammer & dolly methods.
Just trying to be a little encouraging...
https://www.flickr.com/photos/132029468@N08/