Although I haven't put keyboard to code editor yet, I spent a lot of time yesterday thinking about the new feed processing system for my personal feed reader. Back when I built the original feed system and Drupal 7, feed items from the aggregator module weren't entities. Now that they are entities in Drupal 9, they are fieldable, so I can do far more with the items than I could a few years ago, so maybe I can get away with using the aggregator module instead of the feeds module. However, as memory serves. the aggregator module did not handle title-less items very well. That might not take a big deal to some but some of my fields on my feeds have title-less posts. If you can't picture a title-less post imagine a tweet or better yet this post which has no title it is just a body. I'll have to do some testing to see how well it can be handled.
Also I'm giving a lot of thought to whether I want to add the feed reader to this site or if I wanted to leave it on its own Drupal instance. Having it all in one site will make support and maintenance easier at the cost of some performance because I have a number of modules turned on to support the feeds Bajwa that I don't need today. Another thing to consider, if I ever decided to go back to hosting the site at Pantheon or some other cloud platform, I'll have increased fees to host it.
Lots of little things to consider.