Instead of limiting the number of posts one could introduce 'diminishing returns' which means one still could write as many posts as one wishes, but starting from a certain number of posts upvotes on every further posts would have a weaker effect than upvotes on previous posts.

This would work, but it would come with a side effect. If I would create a few shit posts before posting a real masterpiece of a post, the good post might receive less rewards than the bad ones :)

Similarly one could try to prevent the effectivity of upvoting the same users again and again (circle voting). How about if after each vote on a specific account (including one's own account) each further vote on the same account would lead to significantly less curation reward for the voter and less profit for the upvoted account?

As we discussed many times over the years, in my eyes this would be the ultimately perfect solution. Using something similar to the calculated CSI on SteemWorld as a factor, so that the voted rshares would be multiplied by it prior to subtracting them from the pool and adding them to the active votes.

This would not eliminate all cases of abuse (for example, if someone owns many accounts with much SP and votes each day with a different one), but it would work very well for most common cases.