Right now I don't think Proton would do much better than existing options. There are browsers on different ends of the privacy to convenience spectrum (and these are all Gecko based):
- Firefox: decent privacy by default without changing any settings
- Librewolf: Firefox but with hardened settings for privacy
- Mullvad Browser: Almost the same as Tor but not on the Tor network, which is admittedly slow
- Tor: Uses the Tor Network and by default very hardened for Privacy but makes lots of convenience and QoL tradeoffs, including letterboxing to common browser resolutions.
No way in hell would Proton make a Chromium-based browser, the only way that would not be hypocritical is if they fully open sourced it (you could compile it yourself), and maintained their own fork completely devoid of Google tracking and telemetry.