Correct, the highest voting return is to sell your vote because you get the curation value of the vote + money on top of it. Self voting isn't as good, which is why some of the most flagrant self voters moved over to bidbots and got a better return there.
I don't think changing curation rules or amounts will change much. The only real option is community led downvotes of socially and arbitrarily deemed undesirable content.
Changing the curation/author mix reduces the heavy lifting that downvotes have to do. It takes less downvoting to push self-voting or vote-selling down to 50% than to 25%. There could also likely be more collateral damage with the latter. That said, downvotes alone without changing the split may still work. The question comes down to: 1) degree of relying on altruism since they are still not incentivized, 2) effect of remaining downvote discouragements such as retaliation or social reputation, 3) if the free downvote power is smaller than the upvote power there may not be enough to perform more heavy lifting.
Still, it could work.
Best of all is to sell to your vote to someone before the mega bid-bots upvote. You get paid for the vote as well as frontrun for extra curation. You also don't need to have your SP tied up in a bid-bot.
This might be the reason why I get 200 upvotes before I use the bid bots !
That could be part of the reason. Resteem services also dish out 300 worthless votes for 1 or 2 SBD.