I never get any notifications except notifications about them telling me Discord is available, and also comments and replies to stories. However, I am also in the habit of turning off the majority of notifications in Settings the moment I have an account anywhere. Maybe that would help you
I have a feeling these spam notifications are just more so about Tapas trying to do more to promote a series. Even if it's already popular, maybe they feel more needs to be done or something. The biggest series on here are still not popular as the biggest ones on Webtoons, for example, and the two sites are definitely competing. Tapas is trying to step up their game, I guess? Repetitive ads can help something gain views (and once you get views, hopefully readership), The more someone sees something repeatedly over time, the more one "trusts" it and wants to give it a chance. This isn't true for everyone, of course, as others are turned off by having to see it so much, but this happens a lot. I think it's called the "mere-exposure effect".