I haven't tried voice to text on the S10+ yet but I was curious as to how you send that text? My car will read incoming texts to me and on my previous phone which was a LG G6 I tried to use "Hey Google" to respond and it did compose the text but I still had to hit a button on the phone to send it which to me is strange because I thought the whole thing was for it to be "hands free".
I don't think talk-to-text is designed to be hands free. It's just a way to compose a text, email or other message without pecking at those itty bitty phone keys, and to do so at a talking rather than finger-pecking speed. So, I have to hit Send once I compose the text, email or message, just the same as if I'd typed it by hand.