This is not so much an answer to your question about a mid-layer as making an observation: your days (24 hours) will probably divide very distinctly between the time when you are doing trail work (rough, dirty, sweaty) and when you are not doing trail work.

When you're working you'll probably not be needing much extra warmth, because your internal engine will be revved up and burning fuel at a furious pace, but your clothes do need to stand up to some real dirt and abuse.

The requirement of layering for extra warmth will mostly be needed when you are off work, in which case, I wouldn't worry too much about ruining your stuff, unless you'll be bushwhacking in your spare time.