Wow, I gotta say, reading that the DS Mk I is noisy and grainy threw me for a loop. I can say with 100% certainty that mine is neither of those things, quite the contrary. I absolutely would not change DACs if I were you. I'm sure there are better DACs in the world but not at anywhere near what the DS Mk I goes for these days. The dual I2S inputs are still pretty rare and such a nice feature to have. I2S definitely sounds better than other connections.
I have no experience with the AirLens but I doubt very seriously that's holding you back either. Fiber conversion did clean up my signal quite a bit and I have not noticed any of the bad side effects that some claim come with that method. I also run my signal through an Ether Regen (original version). I feed it with the fiber optic. I can't swear that makes a huge difference but it certainly doesn't hurt. I power the Ether Regen with a LPS.
I'm confident you'll get to the bottom of your issue and I don't think it'll cost you an arm and a leg either. If it were me I'd concentrate on the signal delivery to your components, which I think is what you're doing anyway.