This. Scrum is not adequate for this.

What we used to do (while growing the SRE team from ~8 to 30) was having dedicated teams on specific projects. Each had their own scheduling system as they saw fit. AND a weekly rotating support duty which took precedence: teams had to allocate someone from their own capacity to this duty.

The importance was to adequately rotate everyone, to pair a senior and a junior in the support team, and to be available to them to debrief, or to reallocate high priority/criticity tasks to more expert members, if required.

Also, if the support queue was empty, they could do maintenance/exploratory work, but NOT their own team project work.