Repair for Apple Education devices

I work for a UK local government authority that has around 100 schools with Apple devices mostly purchased through us but also by the schools themselves. We currently have a hardware SLA with our schools to repair literally thousands of devices. However due to the security problems of replacing parts, ie on IOS devices the Face ID, Touch ID, battery information functionality is lost. We do not have an issue with only using original Apple parts but would still need access to diagnostic tools to resolve this issue.

Do we need to enroll in Apple's self-servicing account programme, become an Independent repair provider or Apple authorized service provider and have an ACMT and IOS trained technician for this?