Did you know that hashtags in Mastodon have an RSS feed? Just like following users via RSS, all you have to do is add .rss to the hashtag URL.
For example, I like to follow #photography. The URL for that tag on my server is https://twit.social/tags/photography. If you add .rss to that URL like so – https://twit.social/tags/photography.rss will give you access to the hashtag’s .rss feed so you can enter it into your favorite RSS reader and view the feed.
Note – I am on twit.social. Your hashtag URL will reflect whatever Mastodon server your profile is on.