remailable works by associating their email with a device ID in its own db, so you don't even need to ask users for their device ID as long as you can send emails from a unique email addr. (I can explain more if helpful!)
I also have a public free version hosted at https://remailable.getneutrality.org/ if you prefer not to run one yourself, which you and your users are welcome to use. (it's tricky if you haven't worked with AWS serverless tech previously!)
Thanks again, I'm already using unique email addr just to be safe with Amazon and have prior experience with Lambda.
I'm currently in the process of validating my project and if there's enough need for reMarkable support I'll definitely use your project with due credits + donation(If my project makes any money).
I also have a public free version hosted at https://remailable.getneutrality.org/ if you prefer not to run one yourself, which you and your users are welcome to use. (it's tricky if you haven't worked with AWS serverless tech previously!)