I'm a little out of my depth here but I'll offer you what I've managed to find on the subject in the hope that something here might help:
Firstly, Android devices come with a text-to-speech facility built in. See here to enable it:
https://support.google.com/accessibility/android/answer/6006983?hl=en-GB
Then it might be possible to create a text file (using whichever office software you've got on your device), where the text-to-speech facility can be used to read out selected text.
For some other apps which might help, see here:
https://thedroidguy.com/2018/06/best-text-speech-app-tts-android-1070049
Your proposed way of doing things is known as 'phrase banking' but you might also want to consider full 'voice banking'. See these links for an explanation:
https://www.mndassociation.org/wp-content/uploads/07c-speech-and-communication-support.pdf
and
https://www.mndassociation.org/wp-content/uploads/07D-Voice-banking.pdf
Voice banking requires specialist equipment but, if you can't obtain it through your local support services, the Motor Neurone Disease Association has a loan system. Further, voice banking requires the payment of some fees but MNDA can offer some financial support. See here:
https://www.mndassociation.org/getting-support/communication-aids-service/