It sounds like you're going along the right lines with upping the veg, and you're right to introduce new veg carefully in small amounts until they are used to them.
I wouldn't recommend feeding cabbage, broccoli, brussels sprouts and spring greens all on one day though. Spring greens is usually a type of cabbage. All of those foods are fairly 'gassy' and can aggravate bloat in those piggies susceptible to such digestive problems.
You can still feed all of the above, just in small amounts, as part of a good variety, and not all on the same day. Maybe cabbage, pepper and something else as a menu for one day?