All are good points. But you fail to take into consideration the changes in BRs, across the board that would start to happen.
Let’s take your 9.3 vs. 10.3 example for starters. Yes, nations have very strong tanks at 10.3, and while i disagree that 9.3 tanks generally struggle against them (9.3 has been my personal favorite BR to play for a long time), you don’t take into account that those very strong 10.3 vehicles would see their BRs start to go up (or less strong 9.3 vehicles start to go down), which would create an even playing field in the long run.
I’ll take the community favorite target of hatred, the 2S38 for reference. It is readily available to players who have just started the game, and is quite an appealing to said audience. Of course ALL of the statistics for it are skewed in favor of keeping it at 10.3 because of this (low skill, no actual lineup etc.) But use whatever % of top performers in it, and it shows the vehicles true capabilities, thus it would most likely move up quite soon.
(I wont participate in discussions that hint that premium vehicles are kept at certain BRs to boost sales. Just saying it now)
My solution, like any solution, wouldn’t instantly fix everything. But after a short while and a few balancing changes, things would start to shift towards a more even playing field.