Ask HN: Proven workflow for maintenance contracts?

I'm overseeing an agency team who manage several webdev client accounts with ongoing support retainer contracts, all in a relatively similar tech stack and vertical. I'm working on optimizing our workflow with our project managers.

The tasks vary quite a bit and range from very minor cosmetic fixes, to resolving critical issues, to larger tracks of development.

This is my second time giving oversight in this scenario and I still feel as though I haven't seen a best practice for 2 core issues:

1) The correct way to triage tickets from many clients, with varying complexities, across the team.

2) Preventing developer burnout. The work doesn't involve a lot of innovation, mostly changing or fixing things, but some of it needs a pretty experienced developer to resolve. And the tasks often need quick turnaround time.

I feel like this is maybe a solved problem in other industries, but I have yet to be able to find a good resource on the best approach for web dev. Any tips?