How to balance lecture participation and not being considered a show-off?
A key question is - why restrain your enthusiasm?

Importantly, it does not really matter what the other classmates think of you - you are there for the interest in the subject, not for their approval. As you stated, you have not noticed any negative reaction - it could be a case that a few others in the class are benefiting from your contributions - and more than likely, the instructor appreciates your participation as well.

It sounds like you are polite, holding back and waiting for anyone else to contribute - that is a positive.

If you are still concerned, speak with the instructor, ask them if there are other ways you can help get the class engaged and of course, ask if he/she thinks you are contributing 'too much'.

What to do? Don't change, you have enthusiasm, you politely wait for others to answer before you contribute. (But as suggested before, talk with your instructor about your contributions).

Just to clarify, I am speaking as a practicing teacher