VCO sinusoidal output design
You can squash the triangle wave through a nonlinear network of some kind to get a sine-like output. If you need a very clean sine wave this is not a good way to do it.
The venerable HP3311A function generator, which you might want to research for your project, does it like this:

In analog synthesizers this is commonly done with a spare LM13700 section. Not perfect, but totally fine for its intended purpose. Sine shape can be further refined with trimpots in place of R2/R3/R4.


As for the frequency range, you might consider a range switch that switches a different capacitor value into the circuit.