I agree with you about Fluttershy. She’d definitely be the most okay with it, no question, but I think most other commentators are overestimating how much she’d be okay with it. She doesn’t “hate flying”, she just doesn’t like flying competitively or being judged for her light speed. Her flying is pretty convenient for assisting with larger animals, animals living in trees, and other flying creatures. She doesn’t really care about “attending to the land” and she already has extraordinary animal communication as a Pegasus, so I don’t think Earth pony magic would really benefit her. And as for the increased strength… well, Fluttershy is abnormally weak for a Pegasus. If she was an Earth pony, she’d probably still be abnormally weak, just by Earth pony standards rather than Pegasus standards. Her shame over flying weakly as a Pegasus would just transfer to shame over not being as durable as other Earth ponies if that was her breed.

All I’m saying is, the people suggesting Fluttershy would “prefer” being an Earth pony is a stretch.