It seems to depend a lot on the topic tbh, for example cross stitch is more active on Lemmy whereas knitting is more active on Mastodon. Memes, Lemmy. Gamedev, Mastodon. etc etc. I end up cross-posting a lot from Mastodon to Lemmy just to cover all my bases 😄
Your post kind of dismisses hashtags but given that they’re basically what all of Mastodon runs on, I’d very much recommend using them. Both to build your own follow list and feeds, as well as in your posts to reach other people.
And don’t be afraid to ask for a boost if you have something to say that you think is relevant to a much wider audience.