You'll still need the quinine bitterness. I haven't the slightest idea if some sort of quinine concentrate is available or how to add it.
In any case, it looks like OP has some options already out there. Fever Tree is pretty good stuff, if a little pricey (but I would expect to pay more).
Frankly, I'd quit drinking tonic water if I had to drink diet tonic water. I haven't found one, yet, that wasn't awful.
I actually like it without the quinine, you get more of the actual gin's more subtle flavoring, but you are absolutely correct.