Where there's a will there's a way
What about multitasking where they could train up existing staff to drive trains (guards or anyone else suitable) and then utilise them as and when required with an additional payment being made when employed on driving duties.
It's been considered but not possible to implement. If you hold a driving competency you need to regularly drive, just like managers with a key do. Otherwise you loose your competency and route knowledge and need re-training.
Plus there is no need. This isn't normally a problem. As I say, with Sundays outside most weeks there are more drivers wishing to work than there is work available. Infact, Sundays are easier to cover than many week days and people think twice about going sick on a Sunday as they loose money for it.
The problem is with any overtime that if the company upset it's workforce they tend to loose the will to do extra...
But you are looking to fix a non existent problem.
Plus it is very well known that railway companies operate with less drivers than they need. Tryst me, your suggestion to recruit more has been considered but the TOCs find it cheaper to have vacancies unfilled and rely on overtime to cover it all. Far, far cheaper.