I have been following a Lemmy community from Firefish and I only get original posts, you can see this behaviour here: https://electricrequiem.com/@comradeship@lemmygrad.ml
However Firefish has had problems with following Lemmy communities so this may not be the default behaviour from Mastodon.
Still, I do think that boosting all replies is not ideal, especially for busy forums. If a topic gets say 50 replies, it can really flood your timeline. Also personally I have set up my Firefish account to only show me original posts (and boosts), to have a cleaner and more compact timeline. With a forum account that boosts everything, you can't filter replies, they all end up in your timeline.
So my suggestion was to perhaps create two accounts for following a forum, one than boosts everything and one that boosts only original posts. This way users would get a choice, and if they find that following everything makes their timeline too busy, they can choose to get only original posts, instead of unfollowing the forum completely.