If you must:
- Proton Mail/Calendar ➡️ Posteo, Tutanota, Mailbox.org
- Proton Drive ➡️ Tresorit, Nextcloud, Filen, Syncthing, MEGA
- Proton Pass ➡️ KeePass
- Proton VPN ➡️ Mullvad VPN, IVPN, Windscribe
- Proton Wallet ➡️ Cake Wallet, Electrum
Some of these don't have first-party mobile or desktop clients, so here are some apps to use them with:
- Posteo & Mailbox.org - Thunderbird (desktop, Android), FairEmail (Android), Evolution (Unix-like), Geary (Unix-like), Claws Mail (desktop), Fossify Calendar + DAVx^5^ (Android)
- KeePass: KeePassXC (desktop), KeePassDX (Android), KeePassium (iOS)