Well, I think periodically adding paragraphs and adding/removing links on your homepage won't affect SEO ranking, the factors affect your ranking is the external link qualities.The words quantities closely related to your keyword density which you should bear in mind.You can check it via some keyword tool then increase the words quantites while keep the keyword density reasonable.

Your best bet is just to always do your work for the customers, not the search engine, they in turn might backlink to you, or share your link socially. If you make a crappy spammy website, you will never get your real users sharing your product. This takes a very long time to do, but it is consistent growth.

Updating your webpage often is pretty much expected. Each time I do a major update to my website I get a burst of search engine traffic, but I do not update just for that burst, I only update when an improvement was needed, or something is added to the website.

if you are changing your content and adding new, fresh, quality content then it help in SEO.

If you rank a page with specific text/links & remove those same text/links, sure it can drop you in the SERPs.