I’m thinking of using StandardNotes as a notepad, but to encrypt, I log in and create my account and save it in the cloud. However, I would like to save only in the smartphone offline, not in the cloud. https://www.privacytools.io/software/notebooks/
There are several apps on F-Droid, if you use it. One is Markor: https://f-droid.org/en/packages/net.gsantner.markor/
I personally use Simple Notes from the F-Droid store, it’s very neat and simple.
Simply dont create an account and select a passcode, for example. Notes will be encrypted on your device but not backed up on any server.
As you already have an account, log out, saving your notes befor and copying them when logged out. When you are sure your dats is safe, you can erase the account