Re: So whose bright idea was it in the first place?
"My concern would be why is this some sort of bespoke project rather than just what everyone else uses for the bog-standard business practices (ERP, HR and bank reconciliation?"
HR and payroll should be common with other businesses. Rented property management will have commercial packages available, even specialist social housing packages have been developed for housing associations. But managing electoral registers or bin collections isn't something that a regular business requirement.
"Gosh, if only there were other places who needed those!)."
There are. Other councils. That's why it makes sense to sponsor a modular local government administration system; modular because the various functions are distributed differently between different tiers in different parts of the country.