As you could tell from the title, this is my last post on this website. As you could tell from the title, this is my last post on this website.

I think technically there is nothing wrong with what you wrote, but at least in my experience the more common/natural way to write something like this would be "As you can tell from the title, this will be my last post on this website".

As you could tell from the title, this is my last post on this website. As you could tell from the title, this is my last post on this website.

I agree that "As you can tell" is the usual wording

The main reason I am quitting LangCorrect is that I stopped seeing any significant improvements in my writing, and I can not afford to spending this much time on it anymore. The main reason I am quitting LangCorrect is that I stopped seeing any significant improvements in my writing, and I can not afford to spend this much time on it anymore.

My brain keeps stuttering over this part, but I can't say that "afford spending" is wrong exactly... whether a verb takes the infinitive or not is completely arbitrary and dumb anyway lol Seems like using the infinitive is much more common with "afford" (see: https://ell.stackexchange.com/questions/17351/to-afford-doing-something) But I guess it's up to preference.

I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I had've been using this site. I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I've been using this site.

I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I hadve been using this site. I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I have been using this site.

Since you're still using the website to write this post, even if it's going to be your last one, I think it's more natural to use "have" instead of "had" - "had" would imply you stopped using the site in the past, but at the time of writing that point hasn't quite come yet.

I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I hadve been using this site. I really appreciate everybody's time and effort that has gone into correcting my posts over the course of the year that I have been using this site.

I agree that "have" sounds better since you were using the site while writing the post