To succeed the 6th generation SP Delegation projects, the APY should be higher than other generations.

There's a tentative look at potential reward distributions, here and a look at past performance, here. Rule of thumb, I think the Thoth account probably needs to have about 2-3x the stake size of delegators in order to provide decent returns by default (not counting potential downvotes). Of course, as you alluded to, if Thoth attracts organic votes, then the stake size requirement is reduced.

It's definitely the goal for Thoth to provide those sorts of returns, but I don't really think it's necessary because some investors might accept lower returns in order to avoid the daily posting requirement and, hopefully, protect/grow the value of their core stake. IMO, to succeed, Thoth just needs to be near parity with the existing generations.

For small delegators, that's already happening. Unfortunately, I don't control enough stake to support larger delegators at those rates of return unless/until Thoth attracts a larger organic audience.