You are correct inside a single transaction you have no control over the ordering of devices. Actually unless you run out of threads (in which case threads will do multiple devices in sequence) the devices will all be provisioned in parallel all at once.
Nano services as you suggest is one way to go, its a relatively simple plan with two steps no pre-post conditions etc. Note that a nano-service is an asynchronous construct, when you create it from an API, that API returns after the first commit, so you get a +ve response in your case once the OLT is configured. The CPE configuration then happens asychronously and you may want to wait for it to coplete.
An alternative when calling from an API that you want to keep synchronous is to use a flag, say you have a boolean leaf configure-cpe You make one call with the value set to false to configure the OLT, then you make same call with value set to true to configure the CPE.
