You can follow multiple hashtags on Mastodon very easily:
1. Search for a hashtag
2. Go to the search results column, open its options and select "Pin"
3. On the pinned column, open its options and select "Include additional tags for this column"
4. Add all the hashtags you want to follow to "Any of these"
This pinned column will then show results for the tags you add to it.
It allows you to follow topics in a single column the same way you would follow people.