Keep in mind that the output of these feeds will be instance specific, and the tag feed will only get mentions from your instance and instances yours can “see” (or gets by follows with federation). So if you use a different instance, you may see more or less in your feeds. Because of its size and depth of federation, this makes mastodon.social a good bet for these sorts of subscriptions, but your experience may vary depending on your needs.
In addition to following people on Mastodon in your feed reader via #RSS, you can also follow hashtags which appear there. For example, to follow #IndieWeb try: https://mastodon.social/tags/indieweb.rss. #FeedReaderFriday