Hi, Jospeh
Yes, unfortunately, delegation isn't done at a Management Group level.
You can deploy a policy with Contributor or Owner rights to the Management Group - to onboard all subscriptions: You can deploy a policy with Contributor or Owner rights to the Management Group - to onboard all new subscriptions: https://zcusa.951200.xyz/en-us/azure/lighthouse/how-to/onboard-management-group - but you won't have control of the management groups itself.
There is a vote for assignment: https://feedback.azure.com/d365community/idea/db5090d8-f824-ec11-b6e6-000d3a4f0da0
It could be worth a look at Policy as Code - as an MSP, you could stand up a Service Principal in your customers tenant and use that to deploy policies to your management group: https://zcusa.951200.xyz/en-us/azure/governance/policy/concepts/policy-as-code.