A couple things to keep in mind?
Adding anything inline that requires a barb fitting that goes inside the tube will almost automatically reduce the opening and cut the flow to some extent. Unavoidable but something to know first.
On the bubble part, I fully agree that the inline commercial versions are less than I want. As I see it, the problem is one of simple time. The tiny little space given to the diffuser will not let the CO2 stay there long enough to move into the water if one is pushing very much CO2.
The reactor I favor is the Grigg's style as I find it more adaptable to meet the different size tanks I use. I never see any problem with building too large (other than price and space?) but too small will let bubbles flow through and into the tank.
Not hard to build if you can cut and glue PVC pipe. I keep it simple and let the size make them work. Basic idea is to push the CO2 into the top of a pipe, push the water down so that the large bubbles float up against the flow until they get small and pass out the bottom. Too small to see at that point!
Some pictures of my build that I now use as standard design.
Top showing fittings and adapters for water coming in from filter. Also note CO2 tubing drawn through undersized hole to make a good seal.
Bottom where water returns to tank.
Keep it simple and avoid the urge to mod the design and add bells and whistles which may not work?