The problem with a cross-poster is that it encourages people to stay on Twitter and give no incentive to move to using their Fediverse account as primary. But that’s better than people not posting to Fediverse at all. It will be harder for us to convince people to join fsmi.social if all the main accounts of FSMI itself are on Twitter. Cross-posted toots will still carry their Twitter references. Another (even worse) option is setting up bots to mirror the accounts. In both cases, the audience on the Fediverse have no way of interacting with the original poster of the tweet.
Posting individually to both accounts is the best approach but more work. On Android, there is an app called Twidere that supports both Mastodon and Twitter.