I wouldn’t know how to make mutagens as powerful as serums. But.
You could use debug commands to give yourself as many serums as you have appropriate mutagens, and then get rid of the mutagens. A workaround mod might be easy but would take some effort: Basically you’d create custom recipes such as:
Skills needed: none (or cooking 9, or 10, or whatever)
Tools: none
Ingredients: 1 bird mutagen
Result: 1 bird serum
…and repeat for each 20-something mutagen types, or just the ones you’re interested in.
Other than that, you do need serums to cross threshold. You sure Genetic Chaos doesn’t grant you post-thres mutations? I’m not surprised if it doesn’t. On a more rare occasion, zombie scientists can inflict mutations upon you but that’s pretty rare, and mouth environmental protection might stop that (I suspect). And even then, post-thres? Probably not. I wouldn’t know for sure, though.