Is scrum or kanban really useful for SRE teams?
We don't use Agile for the DevOps group ourselves, but we do integrate with the normal Scrum Teams. When something is needed by the team from DevOps, such as optimizing the build server, the related team puts a PBI in their backlog with a 'DevOps' label. Our lead has a custom dashboard in Jira with all issues labeled 'DevOps'. They work with the Scrum Master to get prioritization and then one of the DevOps Engineers is tasked with being an ad-hoc member of that Scrum Team for the life of that issue. This helps us prioritize work based on our "customer's" priorities, ties our effort to the sprint, and allows us to get "credit" for the work we're doing.

Prior to this, we used Kanban on Jira just for a todo list. Unfortunately, we would sometimes have to stop working on something we had planned to work on because one of the teams needed something. Now, unless it's an emergency, they basically request DevOps resources (people/time) via their backlog and the Scrum Master communicating that need to our Lead.