Raspberry Pi expert needed for VNC

We are using a Raspberry Pi 3b+ with 7" "official' screen as the basis of a data collection system. Pi screen displays fine. We have been been remotely connecting via realvnc free account. The remove VNC connections have been fine in showing the actual size Pi screen on a laptop, or large ipad (anything => pi screen size). Remote viewing on a phone is very crude / awkward because of small screen size. We've rewritten our HTML screens to be responsive for phones. We are now on a VNC enterprise account to get their "virtual mode". We assumed this was the answer; but can't get it running properly. so we still don't have a good solution for the small-screen problem. The customers "viewing device" could be anything- we have no control over it. The solution needs to be programmed in for all reasonable possibilities. End user should not have to do anything past setting up their own VNC account. Can provide hardware, or files if we get to that point. For what it's worth; we are in eastern USA