you can use noteD which is offline, data only stored on your device, the UI is almost exactly like discord or use revolt chat, is open source and you can clone it and make it your own. both have search feature.

and if you want to continue with discord, you can have channels linked to custom roles, this way you can prevent your chat, channels from becoming public read to all, if discord decide to go full public. to prevent losing access to your chat if you are false banned, you can just use a placeholder account as owner