@skyscraper great post and fun to think about. First of all you've built a great system (and not just saying that because of our shared good taste). The law of diminishing returns is very real IMHO, and you'll have a tough time getting more than a few % improvement over what you have with this budget. If you did pick up some more space a bigger Magico (A5 or S3/S5) would serve you well. Looking outside of that line others have suggested Joseph Audio, YG and those are worth a look. If I were upgrading my Luxman gear for my Magico's I'd be considering Accuphase, Gryphon and maybe D'Agostino.
My initial thought was much like others, focused on the room and ensuring a dedicated listening space. If nothing else I might collect some rare first edition vinyl.
But I'm going to take the conversation in a different direction that no one has picked up on. You mentioned in your original post the possibility of a Mercedes convertible. If you have no toy/fun car today than my unequivocal vote is to buy that, paired with your current system.