Consistently use modelconfig utils

When creating new models, I've found it helpful to use [helper functions](https://github.com/lnccbrown/ssm-simulators/blob/ef7b67e51a9bd394f74f54499c68c1813eace81c/ssms/config/_modelconfig/utils.py) that streamline creation of model configs, especially when the number of parameters is large.

Is this something that should be used consistently across all models?

_Originally posted by @AlexanderFengler in https://github.com/lnccbrown/ssm-simulators/pull/227#discussion_r2484159742_

Start with ssms/config/_modelconfig/utils.py and compare its helper functions with the model configuration implementations across the repository. Determine whether using the helpers consistently is intended, then document the affected models and the agreed scope before making changes.

Written by the indexing model from the issue text.