Chris, when you capacitively "pull" an Xtal, you lower the Q of the
tuned circuit hence the amplitude of oscillation varies with the
capacitive loading on the Xtal.

This is not so much a problem as a natural effect that requires
compensation.

Arguably the easiest way to compensate for this is to simply drive a
switching transistor or IC into saturation throughout the range of
amplitude variations that the oscillator experiences. Essentially,
you're buffering using a quasi Schmidt Trigger arrangement, which
produces a constant amplitude output.

I believe that the problem that you'll eventually face is how to keep
your 850-Hz shift constant and centered. When I was active in FSK TTY,
my solution was to incorporate a monitor scope with filters into my
VCO, plus provide real-time adjustable shift and centering adjustments.

Hope this helps.

Harry C. (ex K2JEZ)