Is something like write freely or plume what you’re looking for?

I think it should work, I just tried to follow you with my mastodon account and it works fine

If you downloaded QKSMS on F-Droid, QKSMS+ should be enabled by default and is free. The purchase option is replaced by a link to make a paypal donation.

However, QKSMS doesn’t seem be maintained anymore, so maybe you should check QUIK which is a fork of QKSMS.