If I understood correctly you have a Laravel app which will use different microservices and you want to choose Lumen for microservices?
Is there really a need to separate API and main Laravel app? Microservices are great if you are a really large company, however in my experience with small teams, they are a real pain.
If it's for performance reasons, keep in mind that once you add packages like Eloquent to Lumen, it's performance bonus disappears.
You would still need authentication to figure out, which would lose you a lot of benefits of Sanctum, as you would need a separate auth system for api calls elsewhere.
I would rather restructure the main laravel app like this
/project/App -> regular laravel
/project/Microservice1
Controllers
Middlewares
Requests
etc.
/project/Microservice2
etc..
Or any other structure that may suite you. Just add directories to autoloader in compose.json, and laravels DI should work