When trying to promote a comic or novel, you have to think about what you're trying to achieve. What you want: People who will read your work, generating views consistently, and ideally likes and comments too. What you should pay for this: Ideally the only thing you should offer in return is more updates.
The problem is, a lot of people get sort of disconnected from this and overly focused on just the number of subs, feeling very desperate to raise the number to 100 and then 250 subs at any cost even if it's empty and doesn't represent actual engagement that would make unlocking monetisation useful. Having ad revenue unlocked is pointless if your stuff doesn't generate any views.
The best way to use forum promo threads is to post just sparingly, in ones that have a theme, like a specific genre, works with a specific trope, or in ones where you post an excerpt or sample (like "post your best page!", "Post your funniest joke!" "Post your favourite conversation!" sorts of things. Even just "post your latest page!" can be good). Do just one or two a day, rather than a lot at once; people are on the forums at different times, and you want lots of different people to see your promo for the best chance of somebody seeing it and going, "ooh! I like the look of that!"
The worst way to use them is to spam post just your link on twenty of them all in one go, because instead of a bunch of different people seeing your promo, that just means a few people will see your promo twenty times and also think you look desperate and annoying. Threads where you just post a link aren't great, because nobody visits them out of just interest, pretty much everyone is also there to drop their link and go. You can actually check how effective a thread is, by scrolling through and looking at the link boxes. If they've been clicked, there'll be a little grey circle with a number on the box, saying how many people have clicked it. A lot of "Promote because why not lol!" type threads, have a scroll through and you'll find the only person who gets clicks is maybe the OP. If most of the links posted haven't even been clicked, just don't bother.
Sub-for-sub will make your number go up but...

It just makes the sub number go up, without providing you with the hidden numbers you actually need for sustainable growth: Likes (to get seen in the rankings), Views (to generate revenue), Comments (for trending and building a community around your comic that helps reader retention) while saddling you with something to gum up your reader inbox forever.
And yeah, sub-for-subbers will always say things like "Well. It helped me find people who would read my stuff and make friends" to which I will just say... if your series is any good, or you have anything interesting to say, I guarantee you can do that by literally just posting in any thread. Any. Thread. You could make a thread called "What do your characters eat for breakfast and why?" and probably get readers. You could make a thread about "If your series was a sandwich, what kind of sandwich would it be?" and get readers (I'm hungry... I should get breakfast). If you make a series that can't pick up any readers like this, it's not a problem that will go away if you cheese your way up to 100 subs; people won't suddenly start thinking it looks good (nobody can even see your sub count unless they already clicked on it), so the best thing a person can do if they're struggling is to get advice on how to fix it.