The AOSP Keyboard on degoogled Android ROMs don't send what you type to a third party, it does the auto-correction offline. For the Google Board, which is based on AOSP Keyboard, I'm not sure. There are also open source keyboards that have some sort of offline auto-correction too.
However this does not solve the typing on a glass. For that you have Bluetooth keyboards. The problem is that you now need to grab two things to type stuff. That could be a good trade off: you have the keyboard for long messages and can still send quick things on the glass.
Or, you might be interested by the Pinephone with its keyboard case! I have the Pinephone but not the keyboard case, but considering the option. For now I type most things from the computer, glass is good enough for the rare times I don't have my computer. It does not have auto-correction, and I am surprised it does not miss me that much.
However this does not solve the typing on a glass. For that you have Bluetooth keyboards. The problem is that you now need to grab two things to type stuff. That could be a good trade off: you have the keyboard for long messages and can still send quick things on the glass.
Or, you might be interested by the Pinephone with its keyboard case! I have the Pinephone but not the keyboard case, but considering the option. For now I type most things from the computer, glass is good enough for the rare times I don't have my computer. It does not have auto-correction, and I am surprised it does not miss me that much.