Oppressed If you really really need to protect yourself, I would recommend to use Tails instead on a laptop computer. It leaves zero traces on your devices you have even visited Twitter. It is made for activists in repressive regimes.
Tails forces all network connections to go through Tor, and is preconfigured out of the box to not leak any data. It has far stronger fingerprinting resistance than GrapheneOS, and makes it much harder to do mistakes that outs you.
Using Tails, sign up for a new e-mail address, for example at Proton. Do absolutely not use any existing e-mail or your phone number to verify. Use a throw-away e-mail service also in Tails for the verification. It is a bit tricky, but doable. Then sign up for Twitter using that new anonymous e-mail adress. Again, do absolutely not enter your real e-mail or phone number anywhere at all. All usernames and passwords must be new and not similar to any you ever used before. Fake your date of birth too.
If you need to share images or documents, use the metadata cleaner in Tails before uploading. For photos, before uploading, use GIMP in Tails to reduce the image size as much as you can without losing sharpness, and then save without keeping original settings. This hopefully masks the identity of the camera that took the photo, by blurring out unique traits in the static.
If you absolutely must use GrapheneOS, try to mimic this setup as much as possible.
Twitter is known to ban accounts that try to mask their identity, so be aware you might get randomly banned one day and need to create a new account.