I am looking for a way to process beans, or a variety of bean that is as flavourless as possible, to get a "clean slate" for making vegetarian cutlets
In most beans, heat around ~104°C to ~110°C will destroy many of the flavour compounds (a slightly different but precise heat is used for different bean varieties)
Commercially beans are steamed in that temperature range to neutralise them before being processed (e.g. making bean derivatives, or milks)
For domestic processing you could try steaming them in a home pressure cooker to achieve the same effect
Also, many beans are soaked at ~60°C for some hours prior to processing to reduce the "fart" sugars
In my opinion, butter beans(Lima beans in the US?) are very neutral and sound suitable for what you want.
Canellini beans seem really neutral to me. Pinto beans might also work, or black eyed peas.