There's nothing stopping you from posting more and self-voting is also okay but the fact you only post once per day and everyone else in your voting circle does the same looks like maximization for least amount of effort.

You don't always need to vote back as a thanks, leaving a comment saying thanks is okay if they're posts are already well rewarded. If they don't like that and don't vote you anymore in the future then they voted you with the intention of getting a vote back which is the issue in the first place. It shouldn't be quid pro quo voting, it should be voting cause they really like your content and wanted to reward it without expecting something in return - that's what proof of brain is.

Looking at your commenting history it is hard to determine who your friends are when you barely comment:

You must understand that this looks a lot more like you are the same user as the other accounts as you barely have any activity, engagement, votes going to people outside of the circle and in general not being social on a social media platform. You've commented more in the last day than you did in 2 months before that and that only happened cause someone stepped up and attempted to break off the votetrading with some downvotes. That's not the right way to use Hive and appreciate how it works. Look at how other people use Hive, how their voting habits go, peakd.com is a good front-end to check user activity, etc and try and do better. Aim to get votes from as many different accounts as possible and that only works by putting in time and effort to connecting with other people, primarily those who share the same interests as you and in that way it'll make Hive a better experience for yourself as well where you'll enjoy spending time here and not just focus on the rewards.