Unless you have a good tether this is simply a boondoggle. We have a bunch of moon lander and mars lander mission which are higher priority if the taxpayers want something back for their money. 20 miles of tether is a long long long rope. Given the atmosphere a carbon rope might work assuming the chemistry would not eat it up but it would still weigh a lot and require a very large balloon to hold it up. Nobody has built a rope made of carbon more than a few feet long much less twenty plus miles. Given the weight , you are looking at a very large booster in the saturn V range to launch the balloon and the rest to Venus. Program cost would be in the billions.