I would tax only those who want to powerdown instantly - 10% should be fine for now.

Make it optional, if a user decide to powerdown through 4/13 weeks, he shouldn't pay any tax.

As for taxing when buying, I don't see an actual point other then making it a bit different in terms of dynamics.

So, if any dapp wants to powerdown in order to distribute rewards among its stakeholders, it can do it but only if regular power down is used