I'm also at a stage where I don't need any more money, have plenty of donuts, am fewer than 100 items under the limit, and have scarce vacant land to play with.
That puts me (and the OP and a couple of others who've chimed in) in a small but not insignificant minority. We've effectively "won" the game as far as grinding & gameplay are concerned. New content is coming at us much faster than new land. What's the way forward?
I keep the game playable by occasionally hunting down things that can go into storage without significantly impacting the look of areas I'm happy with, and when possible I scale back existing areas that I want to keep. Sometimes I will decide something just isn't good enough and wipe that area clean. At this stage, I think that's all players like us can do. Either we have to find another pastime (including starting a new Springfield if you like this particular game that much), or make an effort to free up both land and space under the item limit.