Excellent initiative. Could we have a "SEP" github label?
For SEPs themselves I think they should just all go in https://github.com/sagemath/sage/discussions/categories/ideas, so a label wouldn't be needed for SEPs themselves since they would have a dedicated place.
A generic "SEP" label for issues or PRs to implement SEPs might be a bit too broad to be useful if there are multiple SEPs being implemented at a time. I guess we could have a label for each SEP, but I think that enabling GitHub Projects on the repo instead would result in a smoother workflow for keeping track of the progress on specific SEPs. We could have a project for each SEP that is being worked on and add issues/PRs to it without an administrator needing to create new labels for each SEP (you need a higher GitHub permission level to manage labels than to manage projects).