I don’t know anything about Proton VPN specifically. So, I’m not sure if you can combine two similar products. Normally the answer is no.
I pay Google for cloud storage and they include with that their basic VPN software that I run on my cell phone and on windows. Running the VPN does not interfere at all with the tunneling software.
Setting security, and everything else aside, except for having an app that can facilitate synchronization over the Internet. I use a product/technology that is free for everyone to use.
It’s called Meshnet. The software does come with a VPN that would compete with your proton VPN only you don’t have to buy it or use it.
The software has a feature that allows you to create an encrypted tunnel over the Internet that makes it look as though you have a private local area network over the Internet.
No modifications needed to routers. No port forwarding.
Each person must sign up for their own free account. Then, each of you can share a computer with any number of other people in your group. Actually there is a limited number it’s I think somewhere around fifty. You’re not granting anybody access to your home network just the syncthing server.
Once it’s all set up, everybody will have their own unique IP address that is on a private tunnel between all of the Syncthing servers.
Think of it as a high tech version of tunneling over SSH without needing to do any port forwarding.
Right now your PC has an IP address given to it by its local DHCP server and a second IP address given to it by the VPN company. This will be a third ip address that is specific to syncthing.
You’ll use the ip address provided by Meshnet for syncthing. When you’re using software programs or browsing Internet you’ll be using the IP address associated with your Proton VPN. The only program that knows about the existence of the tunneled IP address is the syncthing server software.
I have a syncthing server that is always running at home and I also have one running on my Android cell phone.
I can be anywhere, connected to any part of the Internet at whatever speed I have available to me and the syncthing software for Android will have the IP address associated with my machine at home.
I don’t leave syncthing running 24/7 on my cell phone I only turn it on when I need it and within seconds it can find my server at home and begin synchronizing pictures and other files on my phone.
The software runs on pretty much everything. Windows, Linux, Mac OS, Iphone, Android, router.
I have a friend and colleague that is a mac user. I have never used a Mac but we both use the same software and so our syncthing servers can easily talk to one another.
There is one risk. It’s not open source. it’s a free giveaway so it can always go away at anytime.