Like as in beer? No. Or free as in freedom? Also no.
However, iCloud will give you a large amount of the feature set for a fraction of the price (starts at $0.99/mo) and will likely give privacy not too far from what Proton gives you.
In the end, there’s always a risk with putting any information on the web.
The Mac app? That would be great. Only problem is that the plugin ecosystem is centered around macOS, and they use native UI. However, they do have a waitlist for Windows (and iOS).
Something like that is gonna be hard to pull off.