Firstly- my husband's BRP was issued on 04/03/2021 and is valid until 25/08/2023 - what date is the earliest we can apply without causing issue? 29/07/2023?
I see from your previous posts that he came to the UK on a fiance visa. In that case, the earliest date he can apply is 28 days before his BRP expires... which will be 28th July 2023.
My other concern is for the payslips - my next one would arrive on the Monday 31/07 so I was planning to use the one dated Friday 30/06 (and the 6 preceding it) as my evidence - or is that too old? Would it be safer to wait and submit the following week with the payslip dated 31/07 (and letter to reflect that)?
The most recent payslip, bank statement and the employer letter must be dated no more than 28 days before the date you submit the online application.
But if you use the one dated 30/06, then the absolute latest date you can apply is 28th July... however that is also the absolute earliest date you can apply... which literally only gives you a 24-hour window in which to submit the application!
This is not ideal, especially if you are planning to use Super Priority processing, as those slots are only released at 1am each night, which then gives you only a 1-hour window to apply (the earliest you could submit would be midnight and the latest would be 1am) - however, the Super Priority slots sell out almost immediately and you may have to spend a few nights trying before you snag one, but you wouldn't have that option as you would have to get the application submitted by 11:59pm on July 28th.
My advice would be to wait until you have the 31/07 payslip and the corresponding bank statement, and then apply between 1st and 25th August, which gives you a 25-day window to apply (rather than either 1 hour or 24 hours).