The smaller the sizing of the orifice entering the pressure differential will help.

This is due to volume of air vs the volume of CO2 we are adding.
For our applicastions, we are not adding air and lots of it, just a small amount of CO2.

We need a smaller pipe and higher pressure than most venturis. you can modify the orifice effectively using those rigid 3/16" OD tubing they sell at the LFS's.

Using heat/lighter etc, you can taper the tip so that only a tiny opening is present.

This will release smaller bubbles.

The other idea is to place a mister in the venturi lead in and have a loop that feeds water through the venturi section.

Not sure how to describe it yet.
Better make a drawing and post that.

Regards,
Tom Barr