Funny, I've mostly seen complaints that Mastodon is "empty".
Some tips:
- Use high-volume hashtags for discovery, not for following.
- Discovery: niched hashtags (e.g. #photography - >#birdphoto)
- Discovery: accounts that use the high-use hashtags. Follow or mute
- mute/filter "spammers" and "spammy" hashtags
- Ditch FOMO, grow your feed slowly.
- Look at boosted accounts from accounts you follow. Maybe they're interesting for you.
The FB export files have everything (your posts, comments, reactions etc) as html files and images/videos.
Getting it from those files into parts that can be uploaded somewhere else will take some work. Either with programming if OP has knowledge, or copy-paste.