I've never felt like Tapas search function is super effective. Best advice I can give is just to think about what in your story someone might search for- like my story is a fantasy romance about angels and hell hounds, so I tagged "angels", "demons" ect to go with my fantasy genre. Ditto @rajillustration that the episode tags are not searchable, consider them just flavor.
Don't rely too much on sub4sub types of promotions. My advice for on forum advertising would be to keep an eye out for promotional topics (or request topics) that are more relevant to you personally. This is way better than a free for all promotion.
Secondly, put your comic link in your profile! When you engage with the community instead of the promotional topics, people will get interested in what you have to say and check out your work. It's a slow, passive sort of advertising, but this way when someone subs from the forum, they do it out of a real interest in your work!
Thirdly there ARE topics about marketing yourself on the forums, seek out good advice from users with experience!