Ok. I'd like to voice an opinion even though I spec Disc and I haven't progressed to those fights as of yet. (And it might help others if you gave more detail as to what the challenge that you're having with Thaddius and Rassy-25.)
The meta of what I see you saying, the conclusions that you're reaching, and what you're doing to improve is consistent. You are looking to adapt your spec to something that you better fits your play style and gives you a little room to breathe. I can't see the flaw in that.
It strikes me that you're playing on a high enough level now that your attitude and self confidence are likely more important than your "talent points". We see this with top athletes. Yes, some have better stats. But winning the Superbowl is in the heads of the players. This is why the Tennessee Titans failed in the playoffs and the Cardinals went to the Superbowl. One group of players had their heads in the right place. The other group had their heads where they couldn't see sunshine.
Any Advanced Player needs to have their head in the right place first. Everything else will flow. It sounds like you're doing exactly that. Keep going and good hunting.
Oh, and let us know if it makes the preceding fights better, the same, or harder for you.
"If the Truth can be told, so as to be Understood, it Will be Believed." - Terrence McKenna