So currently the faction bank can only be accessed by leaders (giving out money) and everyone can donate as much as they want.
My suggestion is allowing a new donation type which will act as a regular bank. You can store for example 100m in faction and can take it out anytime without leaders around. That of course doesn't let you take more than you put in.
In order to make it scale - the maximum amount can be constrained based on days in faction. Someone who is 10 days in faction can invest much less than 1000 days in faction.
In addition it might be a good idea to give this "storage" bank upgrades too to increase max capacity for each member or for all members combined.or even merits.