Try this hack using Whatsapp:
-Make a whatsapp testbox on your phone:
Create a temporary group
Kick everyone out but yourself
Message something, so the group appears under 'chats'.
Pin the chat and rename it to something like 'test box' or 'clipboard'
-Connect to whatsapp web on your PC
Whatever you throw in the test box from PC/Phone will be ...
You can use a web based text editor like Google Docs or Nextcloud Notes.
I know you said you don't wand to send your data to any cloud service.
But if you run your own service (like Nextcloud), then I guess it is a matching answer.