Privacy setup without Proton

Mullvad takes cash in the mail. However, given what you’ve described, connecting directly to a VPN from your local IP may draw unwanted attention. The VPS self-hosted VPN idea is good. You’re then trusting the cloud provider over authorities in your own country. You can look into confidential computing (e.g. AMD SEV-SNP) to try and make what you’re doing on the VPS more private from the cloud provider, but that adds complexity, and, potentially, cost. 300GB isn’t very big, but bigger than many cheap VPS’s have access to by default. Cryptomator+any other cloud storage you have access to should work. If you have family or friends that don’t mind a used 1TB USB drive attached to a cheap computer and wireguard/tailscale might be cheaper in the long run than paying for cloud storage if you don’t already have access, but that’s work.