Re: HMRC load peaks
You're quite right about the 4 peaks instead of one - the only real difference is it means you're using the capacity all the time instead of just once a year...
As to the issues round using VAT (transaction based tax) info for ITSA or CTSA (profits based taxes) it'll depend on HMRC's cunning plan to get all "records kept and preserved digitally" and how well they can integrate that into the 3rd party software that HMRC apparently expect taxpayers to use in place of their current beloved spreadsheets. Since they can't practically outlaw the use of spreadsheets, the conversion rates to integrated packages will probably be lower than they'd like. As for those (800k or so) taxpayers who can't or won't use "technology" to communicate with the authorities, we're not quite sure what HMRC's plans are.
In any event, the current draft ITSA regs set out reporting periods that are fundamentally incompatible with the vision of the MTD for VAT Legislation Overview to retain existing Prescribed Accounting Period rules for VAT. Until they sort that out, there's not that compelling a case to align your record keeping/submissions for the different heads of tax anyway - especially if they're not going to mandate any other MTD for Business until VAT has been shown to be a success. [Although that is 'shown to the satisfaction of HMRC & Ministers', which may not be the same thing as 'is']