regarding portainer I've got two things...:
1) There is already a pretty handy feature in portainer which enables updating containers very easily! That alone is a feature which should everyone prefer portainer over OMVs build-in docker utility. In portanier GUI click: Containers -> Select Container -> Click “Recreate” followed by “Pull latest image”. That pulls the latest image for the container and redeploys the container with that latest image. So you won't be needed to a) reconfigure the docker one by one with every param. nor b) use tools like watchtower
2) Actually more like a question, is it preferred to run portainer itself also as docker image? Why not install it natively, is there any reason?
How is "Install Portainer" in OMV5 done, installed natively or as well as docker image?