Our expected OptaPlanner implementation would be managing 20-25 constraints (hard and soft) on a data set of 100 employees-clients and 600 visits to be assigned.
With your experience, would it be feasible the usage of AWS Lambda knowing the restriction of 15 mins? Could we get a reasonably good result with that limit if we stop OptaPlanner when it goes beyond that time?
OptaPlanner Benchmark to the rescue!
Seriously, run OptaPlanner Benchmark (see OptaPlanner docs) for 30 minutes and look at the "BEST_SCORE" graph to know what 15 minutes or more or less will get you on your machine. Then rinse and repeat on a cloud like OpenShift or AWS.
As for AWS Lamdba (serverless): OptaPlanner Quarkus is your friend.