The accrual you describe (a large spike at the end of the tax year after promotion, and a smaller spike at the end of the next) is exactly how AFPS75 works, and keeps working even after you switch to another scheme. They can’t and won’t retrospectively flatten that out, it is a ‘feature‘ of the tax treatment of the scheme, and so I don’t see how they could justifiably treat the spikes differently if recalculating the 2015-22 period. All that can be done is to carry forward unused annual allowance from the 3 years before the spike. These are Treasury rules, not MOD: no chance of them changing for the better.
I reckon anyone who was promoted to wg cdr or above after 2013 needs to do some very hard thinking about their 2022 choice; increasing the number of years accrued on 75 makes those tax-attracting spikes even bigger!