You really want to take advise from an IFA before you do anything pension wise - on the tax forum we only really advise ref the tax and there is much more to think about when it comes to adding tp pension than just the tax !!
Having said that if your taxable pay at tax year end is £25,224.12 and you have no other adjustments to taxable income then your calcs look spot on ref 20,179
I'm curious about the rule where you can use a previous years tax relief allowance in a current year.
Since I'll be in the 40% tax bracket next year, would it be better for me hold back my contributions until the next tax year?
For next year would that effectively mean I could add this years unused allowance in next years contribution, and benefit from 40% tax relief from a max of 40k for 2021/2022 and 40% on 25224.12 for 2020/2021.
The logic of your calcs here seem equally spot on - much better to get 40% tax relief on the contributions you make rather than 20% - so everything else being equal delaying payment seems close to being a no brainer - there is though the knock on implications of then having less unused allowance for the following year (year two) which could limit somewhat your payments at a later date if that years excess was used at a later date - ie if 3 years after the second year you wnated to take advantage of that years unused allowances they would be less and year 1 would have fallen out of the 3 year window - i am not an expert on the 3 year widow so dates might be slightly wonky but hopefully you get the principle involved.
Note if taxable income is over 100k you would normally lose personal allowance so practicably speaking tax relief on payments for income in the 100k-125k(approx) bracket is actually 60%!!