If there is a way, to create a role, that can VMUser manage their own disk (Remove/create/modify)? They can create disks but all disks will be limited to a size limit (e.g. 30gb).
You can do this with a trick. If each users has its own lvm group a user can then allocate its own disk. For example. You have a lvm volume and add a group with the amount of 50 GB. This volume can than be added in proxmox as storage. At last you give permission to this volume with the user rights. Then a user can do what you want.