@Matt F Yes, there is a validation around the limit currently when you call compose option. So, the limit cannot be increased with the no. of custom model ids that can be included in one compose model id.
You can use compose model ids in a new compose model id but the limits still apply in this case too. For example: If you use X compose model id with 10 different model ids for a particular form type and then if you would like to create a new compose model id Y with X and another 10 custom model ids of a different form the new compose model id Y contains 20 custom model ids which also includes the initial set. This functionality does help you isolate the custom compose models for a particular form type but the limitation cannot be exceeded.
We will pass this feedback to the product team to check if there is a possibility of increasing the limit in the future. Thanks!!