I posted a similar question in the past. I'm trying to construct an A/B test to compare proportions between a test and control group. The historic proportion in the control group is 43%. I tried to boil this down to a simple hypothesis test:

H0: Ptest = Pcontrol

H1: Ptest < Pcontrol

But, I'm concerned that the desired effect size (maybe a 1-2% difference) might be too small. Thus, requiring a sample size that's not feasible. I'm trying to think of alternatives. Maybe a Bayesian model? I'll have to brush off the dust. Any suggestions would be appreciated.