I appreciate that you want the counter to do double duty.
However from the ergonomic vantage, it isn't so easily done, as the sewing machine height needs to be lower .... the cutting or work table height should be higher.
A good adjustable chair, might help you. I use an adjustable drafting chair ..... it's lowest is the highest of a typewriter chair. The only problem with setting the countertop higher, sewing on it, and having the chair higher .... you may find it too far to the ground to the foot pedal.
Thinking out loud here ..... wonder? could you set the machines into the countertop .... and have a lift up lid/countertop for when you are sewing, and then drop it down in place when you are using it for other purposes? (not sure if there'd be enough height difference?)