There is no such thing as serious/non-serious learning.

I take weekly music lessons and have been doing that for a decade. After every lesson -- since the very first one -- I am amazed by how many simple things that I get wrong, and how many different areas where I could improve. The teacher just sees/hears that immediately, when you have no idea what you did wrong. And the music just sounds different.

Not only that, the teacher discusses the piece with you, tells you the efficient way to practice (a specific piece or specific passage) etc.

You would probably sound ok to a random stranger, but you quickly hit a bottleneck. You spend a lot of time doing incorrect/inefficient things without knowing it.

$100/hr is expensive, but well worth it.