The first question you should be asking yourself is WHY you want a full frame camera.
Full frame are good at some things. But APS-C cameras like your 60D are better at some other things. There's a ton of hype and a great deal of BS surrounding full frame that you have to plow through. Be sure to do a thorough job of it before you decide because simply changing formats has never made anyone a "better photographer"!
Second, why do you feel you need to change brands in order to "go full frame"?
Are you aware that the Canon 6D Mark II sells for EXACTLY THE SAME PRICE as the Nikon D750? Both are currently $1500. Their specs are very similar, too. The Canon has a slightly higher resolution 26MP sensor. Buying Canon instead might allow you to continue to use lenses and accessories you already have.... 6DII uses the same battery as 60D, for example.
Unless you need full frame to make really big prints (bigger than 16x20") or use really high ISOs a lot (25600 and up) AND are okay with bigger, heavier, more expensive full-frame-capable lenses....
My advice would be to take a close look at the Canon 80D (APS-C like your 60D). It would be a significant upgrade:
https://cameradecision.com/compare/Canon-EOS-80D-vs-Canon-EOS-60Dhttps://www.imaging-resource.com/cameras/canon/60d/vs/canon/80d/https://www.apotelyt.com/compare-camera/canon-60d-vs-canon-80d$1000 would get you a five year newer 80D that can use any and all lenses and accessories you already have. True... it's not full frame. But it's also better than full frame at some things and superior to the full frame models you're considering in some specifications. Depending upon what you shoot and what you do with your images, the 80D might actually be a better choice for you than ANY full frame camera.
D750 and 6D Mark II are fine cameras.... Full frame has some advantages. But it also has some disadvantages. The vast majority of users are actually better off with APS-C cameras... which give them greater choice of lenses, lighter/smaller lenses, "more powerful" telephotos, lower cost lenses and more.
Switching brands is always expensive. Switching brands AND changing formats at the same time will be doubly expensive. And Nikon lenses and accessories... on the whole... tend to be more expensive than Canon. Compare their respective 70-200mm lenses, for example. Canon offers four models ranging from $600 to $2100. Nikon offers two that cost $1400 and $2700.
Do what you like and what makes you happy. Just do it with eyes wide open, realistic expectations and only after considering all the actual costs now and in the future.