Proton Apps exclusively only via Tor
A function would have to be developed in the Proton apps that allows you to connect to the apps only via the Tor network, log in and retrieve the respective data.
In other words, the apps would exclusively address the TOR hidden service and never communicate with the clearnet.
This would represent an enhanced security measure. It should be an opt-in measure.
The advantages of this feature are described under the following link:

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privacyFirst commented
TOR network connection for Proton desktop apps would be a great option. However, if one already has one of the paid options for Proton VPN then it would be simple enough to use the Proton/TOR profile, although this is not always desirable of course. So there's definitely a case for this option, as it would be a great privacy and security enhancement.
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w6yx5 commented
The idea that the clients communicate exclusively in the TOR network is a good one. This would make communication within the Tor network invisible to the Clearnet. The only thing that a theoretical threat actor could see would be the connection to the Tor network.
This would allow the full potential of Tor to be used rather than "only" using it as an IP-anonymizing network.