Proton should implement post-quantum hybrid TLS both on SMTP and HTTPS.
This has been normalized with https://www.rfc-editor.org/info/rfc10024/
For SMTP TLS is the ONLY layer of defense when exchanging with other servers.
This is not a substitution for post-quantum end-to-end encryption but another important layer.

It is critical and urgent to implement quantum resistant algorithms in all proton services (VPN included) as other providers (eg. Google: https://blog.google/innovation-and-ai/technology/safety-security/cryptography-migration-timeline/) are moving up their transition due to recent developments in quantum computing.

I generally prefer Proton VPN over Nord VPN (having used both). However, Nord VPN has a "post-quantum encryption" feature that Proton VPN does not. This does not mean that the user's CPU needs to perform quantum encryption, simply that the encryption used is resistant to future quantum decryption, which is of course a far less onerous requirement on a user's CPU. Would it be possible to add a similar feature to Proton VPN?

I would use Proton mail instead of Tuta mail if they added this

Perhaps not entirely. Symmetric encryption can become quite frustrating, particularly when dealing with issues related to the same server.

Also have the Fully homomorphic encryption system that Cornami is developing.