My sympathy to the OP!
I have long felt much the same, ever since retiring, in fact, and the sense of freedom in carrying a camera with a single lens stuck to it is worth much more than the sense of dammit! shoulda brought whatever as well!
This, along with many years of equipment juggling/changing that have cost a small fortune, has led to a philosophical change which is: sell nothing and keep everything. You are better off making a daily choice from what already exists in your armoury than wishing you hadn't got rid of the thing you suddenly find you need. Kicking yourself in the ass is a difficult, demanding process.
What you are not using is not costing you anything extra. It cost you what it's going to cost you the day you shelled out for it. Don't think of it as a permanent piece of exchange value, a negotiable instrument: it's a photographic one. Unless you screw with it, it will probably remain as good in a hundred years as it is now, which can't be said for yourself. Which leads to the next point: stay with one system and don't be misled into buying stuff that has built-in obsolescence. That was one good thing about the Nikon F mount, at least until the advent of the G lenses. Sadly, my stinking old eyesight has forced me into af if I am off tripod and want to be sure, and I usually am off tripod.
But, all is not lost: with the 50mm G, used on an FF and a DX body, I get two focal lengths in one (more or less): a 50mm and a 75mm which actually makes quite a difference in reality.
Money not an issue, the only other thing out of the sytem that I'd buy would be a Leica M (digital) of some sort, just for the experience. I'm sure that it could never be my single system because I enjoy shallow DOF and for me, that means I need to get a good idea of what's what on a screen. No, live view is out of the question: I would require speed.
We all get fidgety feet: there's even an old jazz number proving the point. Resist temptation and, if you can afford it easily, consolidate down to a single system, preferrably a full-frame one for its potential... In all honesty, if it's just for fun, why would anyone need more than a single system? This may appear to fly in the face of my opening bit of personal equipment wisdom, but it doesn't: once you settle onto a system you are set for life.
Of couse, if you are an equipment junkie, then God help you: you are your own worst enemy.
Rob