I'd just reduce the capability of family gold in general.
Investment should only be limited to your estate. Too often family gold is used to funnel gold from one continent to another. May the way it is used should change completely. Maybe instead of family investment increasing the gold output, it should maybe used to increase loyalty and control so there is no financial gain from using family gold but to help your realm overall by aiding them in stabilizing unstable regions.
Or be used to get your new character off the low honor and prestige status quickly. When you spend a large sum of gold, you can quickly gain honor and prestige to get to the point where you can change to any class like cavalier.
Or like Vita's idea, after getting rid of the inability to read letters in prison, maybe increase the time you are held in prison by a lot. Like 2 weeks if you are a member of a wealthy family. If you are a new family you are probably broke so get them released sooner (maybe after 2-3 days. Same goes for poor families as well I guess.). After all medieval people held other nobles to get ransoms and some people had to be stuck in a place for a very long time because they refused to pay. Maybe this will make people less likely to send thousands of gold to their families.
As for control, maybe lower your control over a region, the faster TO should be conducted. Core regions requiring 100% while regions that don't even have a lord and lack any control should only require 20%.
As for war, it should definitely be overhauled. I'd try to make people fight more often but shorter wars. Make recruits lose training and morale as the war progresses. Make training and morale don't recover until the war ends. While there is a war going on, then every time RCs produce new recruits, make training and morale drop slightly. The longer the war, the more drop they suffer since the penalty accumulate over time. Maybe if a war lasts for a very long time, RCs should produce less recruits as well.
