On the commercial dip-n-dunk E6 and C41 machines I have used, the time between baths was 15 seconds. C41 with a developing time of 3'15" was always the hardest one as the film was usually in the first developer for 3'35" which is getting close to a ½ stop push.
The thing is, if you wish to develop your dip-n-dunk machine, you don't have to alter much from what I can see, just ensure consistency and temperature stability.
One thing our commercial dip-n-dunk machines did have, were blasts of Nitrogen every 10 seconds for agitation.
Perhaps you can have a little lift of the film reel or holder by pulling it up say 4cm then it just drops back in and hits the bottom where there is a small glass marble on one side making the reel go sideways a bit, then immediately it starts pulling up 4cm again. This may be a way of getting some simple agitation happening.