It's disheartening, but you can fix that without additional investment even. I've recently joined a free curation group (steem engine) and you really get a lot more interest in your posts that way. When you post in their discord, you have to upvote and comment on 2 other posts.
That said, I'm also buying more SP, it does go kinda slow otherwise.
Good call one buying more SP, its getting cheaper and may go down further still (though I wish otherwise). I think I tried SteemEngine some time ago, is it these guys who take a vote from you everyday?
I have joined their Discord channel. I'm fine upvoting other's content just as long as its worthy.