Sizes: not a significant changes, all LF modes are still about same.
Wall covering with more thick plasterboard will slightly change LF and midrange absorption.
Indeed simulation and measurements will not show anything reasonable for double layer. Here you are fine with more thick layers/wall weight.
Why I wrote here is that I see your before/after REW graphs with damn big amount of VPRs that does not eliminate main problem.
They absorb a lot of energy in 150-1500Hz, while leave a big and untouched bump on a region where you have main problem - quite low frequencies around 50Hz which you can eliminate only with a tuned bass-traps.
Damped 150-1500Hz: that is important area where you could have natural decay if sidewalls will be inclined and floor plane will be non-rectangular.
Interior items like chairs, sofas, humans, carpets, furniture do absorption in that area, and once your finish your room it will make additional abortion in this area, and you will get real problem that room will be overdamped in a specific frequencies.
Means you need to add a lot of diffusion with a different amounts on a different frequencies, and it is not a simple task and it will not sound natural. Looks that now you create a problem which you do not see now, but definetly will see on next steps, and remove initial error will be almost impossible.
What I see on your graphs say me that you need to replace ALL your VPR absorbers with the ones what will work on a problematic frequencies.
Do not kill me if I say that you need to change geometry of walls, accept the losses for building walls and specially produced but unmatching panels and start everything from the beginning
It would be really disappointing if, with such healthy budget and your very detailed approach to every nuance, your room will not be the best of the best.
