Disclaimer: I am NOT a security expert, do your own research and do not blindly follow my advises.
-Use a zero-trust policy.
-Use the KISS-principle. (Keep it simple and stupid)
-I only recommend TOR, I2P or zeronet for folks with a high threat level. (A VPN company can get compromised)
-I do not use any cellular connection on my phone, instead I use a mobile hotspot with web-based SMS function. (It prevents the baseband obtaining the phone's IMEI, etc, it is easier to dispose of a hotspot instead if neccesary)
-Only use verified free and open source software, use the AppVerifyer app and the Accresent and Obtainium apps as a source verification method and application repositories.
-Turn off Bluetooth, NFC and Location.
-Get offline resources like maps, (Organic maps) Wikis, (Kiwix) chats, (Briar messenger) a survival manual, some offline entertainment like games (verify all rom (no-intro) files (datomatic) and check for viruses, use RetroArch)
-Use the internal firewall to block all apps that do not need internet.
-Set autoreboot to 10 min.
-Do not use biometrics.
-Set USB port to "Off" (Charge when phine is off)
-Turn on hardened memory allocator for all apps. (MTE)
-Turn off bluetooth after 15 sec.
-Turn off WiFi after 1 min.
-Set Native code debugging to blocked by default.
-Turn off WebView JIT.
-Restrict DCL
-Use secure app spawning
-Manually give camera and mic access.
-Location: Turn off SUPL, PSDS, WiFi and BT scanning.
-Make a secondary profile for outdoors with less sensitive data on it. (In case the phine gets napped from your hands, the main (sensitive) profile would still need a password)
-Use the duress function, put small note with duress password in phone sleeve. (Not in a way that it is too obvious)
-Manage "open links" on all apps, and manage all permissions of each app manually.
-Use a web-based email account, do not install any app on the phone that can recieve files from strangers.
-Do not use the dialer or SMS.
-Set dialer to "block unknown callers".
-Turn on MAC-randomization.
-For Orbot, use " always-on VPN" and "block connections without VPN" in the main settings menu.
-For Orbot enable "isolate destination addresses" in the Orbot settings menu.
-Give Orbot and Briar Messenger unrestricted acces to battery and internet.
-Use an app like Droid-ify to put really sensitive files in an encrypted container. (Use a different password)
You can make camera pictures and notes from within the encrypted environment so it will never store outside the container.
-Use Nostr for relaying public messages.
-Only use offline AI's and speech recognition. (ChatterUI, SDAI, sayboard for voice to text)
-Use 2FA where possible, onlu use keyboard dictionary to copy them. (Magikeyboard in KeepassDX)
-Never copy/paste sensitive credentials to the web.
-Do not store passwords online.
-Minimize cloud usage.
-And now I need to go to the grocery store before it closes, sorry folks, I hope I did not miss anything, I am in a bit of a hurry.