Welcome to the community Nick 
Here’s Hugo’s (Monzo’s Head of Design) comments on custom timeframes for the Targets -
https://community.monzo.com/t/set-start-of-month/1144/4?u=alexs
There is a solution in the works, although we don’t know when it’ll be implemented yet, it might be this card on the roadmap 
https://community.monzo.com/t/targets-custom-timeframes-1/6242/2?u=alexs