Currently the lambda in charge of scaling up nodes (including Spot configurations), uses the RunInstance API to create a Spot instance. Spot instances as spare capacity might be limited. Best approach when using Spot instance is to diversify across a set of instances that qualify for the workload and use one of the API that allows for that diversification.

The suggestion here is to change the RunInstance call and instead use the Drop In replacement API for EC2 Fleet in instant and the Spot Capacity-Optimized allocation strategy. EC2 Fleet allows diversification and still provides a synchronous API that adhere to Spot best practices providing the spot instance types that are selected to minimise the frequency of interruptions for the workload. More examples here.

Another thing for consideration in the implementation is to use the newly released attribute based instance selection