I found a "workaround" for now:
You can get "somewhat" around the sending issue. Type enough so the "Expand Text Box Arrow" is usable (or move you finger around the text box a little (up works very often), for some reason the arrow shows up then too), press it and now you can press the blue send button to the right. It is far FAR from working / good but it works for now. That also allows selecting and then "overwriting" the current text. That can be considered as a "backspace" workaround.
ALSO: Signal-Desktop uses more battery then it should. It uses a far less if you close it and reopen it and don't open any chats. IDK. why.