Some more thoughts about Yoda and his relationship with the Rebellion:

Overthinking this a bit, I guess I have to agree that it makes very little sense that Yoda isn’t more actively involved with the Rebellion. Sure, he’s not a warrior like he was in the Prequels - but so what? He’s a Jedi with the ability to see the future (at least partially). He could be an invaluable asset for Rebel military intelligence. They could ask him to use the Force to ascertain the probability that some mission will succeed or fail, or use the Force to predict what the Imperials will do in certain situations. The ability to see the future (even if it’s not 100% reliable) would be invaluable in terms of military strategy and intelligence gathering.

But this is supposed to be a fantasy and Yoda is an archetype: he’s the wise master that lives in a remote location and will semi-reluctantly train those who are willing to learn.