To access paths like /opt/ros, you should be able to use an appropriate system-files. Qt GUI applications should also work fine in general without classic confinement - I see you are already plugging the desktop interface - can you elaborate more on the errors which you see? Perhaps you need to ensure you include xcb within the snap itself - in which case I suggest you look into using stage-packages.
As such, based on the above classic confinement does not appear to be required for tesseract-ignition from what I can see.