It'd be really hard to do without getting a feature on the front page, so it's sensible if you're trying to get 500+ subs (and to move beyond it) to first try to make something that looks like it could go on the front page to maximise the odds of that happening. This means that for novels especially, your cover and thumbnail becomes hugely important, and to a lesser extent, your blurb. You want to make them as polished and professional as you possibly can within your available skills and budget.
My advice is don't bother with sub-for-sub. It's an awful lot of effort for meagre sub gain and often a lot of dead subs. You need engaged readers for likes and comments to give you visibility and organic growth. The thing you should give readers in return for their sub should ideally just be an excellent, entertaining story. If your comic or novel is really fun to read and gripping or funny, you shouldn't need to pay people with a return sub for them to read it. Ideally, your thumbnail, cover, blurb and content alone should be so good that even just posting those somewhere without any further incentive should result in you gaining readers. This is a good idea for people aiming for the front page too, because it may not happen often, so you need your time on there to result in the biggest increase in engaged readers possible with a really eye-catching cover.
By all means, use the forums for promo, along with social media, but if people never click on or start reading your work purely based on just seeing it on a thread, or thinking you have interesting things to say about writing, art etc. your response should be "hmm. Maybe my cover/thumbnail/banner/blurb need to be more clear/attention-grabbing!" and then to put some work into improving them so people DO click on that link, rather than saying "Hmm, people aren't clicking on my work based on the cover/thumbnail/banner/blurb.... I know! I should offer them a reward for subscribing!"
Obviously I wish there were more opportunities for our covers and thumbnails to get seen on the site, and I hope Tapas changes some of the algorithm or layout or something to help with that soon. That's out of my hands though, and whether they do that or not, the best thing I can do for my comic's continued growth is to keep trying to make sure I have strong presentation and trying out marketing strategies to see what works best.