Agreed, I usually appreciate someone finishing my sentence if I’m really struggling. But, everyone is different. OP, first off your an amazing person! As for your question, honestly, I would just go by feel. You seem to be getting to know this person relatively well, so base what you do (finish sentence or wait) on a case by case basis. Sometimes it may be right to wait (maybe when chatting) and sometimes to finish the sentence (when placing an order). I don’t think you should ask, sometimes that just causes more awkward embarrassment.