If the Five Eyes countries and their intelligence services are in your threat model you've likely already lost.
Are you saying ProtonMail has a US back door?
ProtonMail is nominally based in Switzerland, so I would assume that any backdoor in their software is controlled by the Swiss.
Although I see their CTO is based in the US so it is certainly possible.
Probably a poignant time to point out that CryptoAG was based in Switzerland for half a century and highly trusted around the world yet controlled by the CIA the whole time.