I don't have an answer, per se, as it's a difficult task and everyone finds readers/subscribers in different ways. But I think the most important factor to consider when planning an advertisement/promotional strategy is how to:
1.) Get your story in front of the "right" eyes, &
2.) Do so in an appealing and effective way.
Point 1 is why sub-for-sub isn't very effective, as you've seen. The eyes that you're getting on your work by doing that are eyes that usually couldn't give a crap less about your story and just want your sub number on their series, much like they're just giving you a sub number on yours. To find more people that actually care about your series you'll have to find other methods of reaching out.
Promotion threads here on the forums are... okay, but also not great. They're also areas where most people are more concerned with simply dumping their link and hoping that people check it out rather than actually taking interest in looking at the other links available. Pseudo-promotion threads are a step in the right direction, those that encourage you to talk about and discuss your story, but also allow you to link it if desired. Relying too much on these forums can be dangerous, though, as you have to remember that most of the people here are also creators. Finding a network of creator friends can be a great way to kickstart your series and get an initial subscriber base, but ultimately you'll want to grow beyond that and find reader-readers that want to invest in your series as well. As you've also come to realize, being a creator is time consuming and while it is possible to support your friends' stories, there's not always a ton of time to do so.
So the next step is looking at other places that you can get your series out in the open and try to draw eyes to it. The site itself is the default option- every time you upload you appear in "Fresh" and that's an opportunity for people to see your work. If an upload does well enough it can sometimes break into "Trending" as well.
Social media and other similar sites are popular options that vary in effectiveness. Sometimes you can strike gold, but often it's hard to be noticed.
Diversifying your content has some potential- for example video content is very popular, so investing some time into making that can potentially get more eyes on you in general, which you can use to try and direct back to your work like a funnel.
Successful promotion is hard and requires some thinking outside the box, and again i don't have any solid answers as I'm still learning myself but best of luck!
edit: realized I didn't directly talk about "promotion in an appealing way". Things like nice promotional images on social media vs. just spamming update links, for example. Or participating in forum threads that encourage discussion and community collaboration vs. just dumping links in promotion threads. And spending energy to make other types of content like the aforementioned video content are examples of what I mean.