I'm actually in this exact same boat (I ported my US number to twilio when I had a long travel stint abroad to retain access, have since built a forwarding system to keep using it).

I registered with the IRS to get an EIN for my "sole proprietorship" (i.e. me), and that seemed to satisfy twilio for the brand registration requirements. Still waiting on a final A2P review for my use case (sending messages to family and friends) so not out of the woods yet, but hopefully that's the last step.