I haven't used Proton, but based on my experience with other cloud storage products and seeing other people's reports thereon, it looks to me like you are running into the problem that there are two different use cases for them with opposite requirements:

1. Local storage is cheap, I have lots of it. I want multiple copies of my data so that they will serve as backups. I want everything in the cloud to be copied locally. (I'm in this category.)

2. Local storage is cheap if you look at the price of external hard disks, but my phone or tablet only has a limited amount of it, and no way to upgrade without buying a whole new device, so it is scarce for me. I want nothing in the cloud to be copied locally, unless I am specifically using it on this device. (Sounds like you're in this category.)

Half the people think the first use case is the obvious one. Half the people think the second is the obvious one. And that includes developers, who will design the product accordingly without giving it much thought. So either half the users get a nasty surprise in the form of running out of disk space, or the other half risk an even nastier surprise in the form of data loss.

Obviously, computers pester us with too many questions these days, but I think this is one case where the software really does need to ask the user which behavior they want. Perhaps with a hint based on how much local disk space seems to be available.