Gayan I think giving user the control is the best solution here.
That makes sense in the abstract. But the GrapheneOS project does not operate in the abstract. Replacing the 80% "charging optimization" feature shipped by Google in AOSP would require development time, testing time, and then rebasing time and re-testing time for every release. The GrapheneOS project is focused on security and privacy, so features that are not security features or privacy features tend to be prioritized behind those. I suspect that if Google ships a user-settable threshold into AOSP then that will be shipped by the GrapheneOS team but that otherwise it is unlikely that a user-settable threshold will be developed and maintained by the GrapheneOS team.
Please note that I do not speak for the GrapheneOS project.