Disclaimer: I am a member of the BuildTeam - the wonderful guys behind MinnowBooster. I use the service (usually) to gift friends upvotes when my VP is low and I have SBD laying around.

First of all, upvote buying bots exist for 3 reasons:

1. Authors want to buy exposure to push content into the trending section of a tag they want to be noticed in.
2. Stakeholders selling votes are basically recycling the SP they would get from curation into SBD. It's like powering down without powering down or losing SP.
3. People / groups running the bots get a cut of 10-20% (as far as I know) of the profit the stakeholders make.

So if we want to eliminate voting bots, or discourage use of them, we would have to remove the first 2, and the 3rd will follow.

Fixing post discovery will eliminate the use of upvote bots for discovery. Since, as you said, vote buying on its own gives little in terms of liquid profits, it's the visibility that drives most people to buy votes. Fix post discovery - incentive to buy votes is gone.

Better rewards for curation will fix the 2nd issue. If we find a way to shift the power in favor of curators and curation, it might be more profitable for major stakeholders to trail quality curators for the SP than to sell their vote for SBD. I would like to see whales paying curators to earn curation for them, rather than minnows paying whales to get some exposure to other minnows.