Investigate usage of marshmallow library

Check if Marshmallow https://marshmallow.readthedocs.io/en/latest/ can be used to either **replace and simplify** some implementation in the current tools, or to implement new improvements.

Such as:
- Schema creation, checking, etc.
- Validation of data
- Conversions between data (YAML) and internal representation (like dacite)
- Merging/Splitting/Comparing etc.

Review the current tools and the README, then compare their schema creation, validation, YAML conversion, and merge/split/compare behavior with Marshmallow's capabilities. Done means recording whether Marshmallow can simplify existing implementation or support a clearly defined improvement, along with the affected tool areas and next steps.

Written by the indexing model from the issue text.