Lots of answers, some humorous, but this is the complete answer, with page references to the manual. There are several ways to do this, and all involve reassigning functions to other buttons.

You can assign exposure lock to the shutter button. Custom Settings C1, Shutter Release Button AE-L - set it to ON to lock focus when you half-press the shutter. This is independent of how you have implemented BBF. (Page 336)

You can also use custom settings F2 to reassign the Fn button with a little more flexibility - you can press and hold to lock focus, press and release to lock focus, you can press to lock focus and exposure, etc. (Page 356) And you can assign the Preview button to do the same thing as the Fn button. (Page 372)

If you are having trouble reaching the AE-AF button on the back with your thumb, consider reassigning AFon to either the Fn button or the Preview button, then using the other front button to do AE lock. These two buttons might be more accessible to a smaller hand than the back button.

And no, you don't need to trade in your D750. At least not until it's replacement becomes available.