I believe volunteering is ethical for both the person doing the volunteering and person/organization "receiving" that volunteer work when:

1. No actual or hypothetical paid position is affected

2. The volunteer gets a benefit equal or greater than what they put in

I believe these principles apply even if the volunteer work is for a private company or powerful university. Volunteer work for other causes can also be ethical even when these principles are violated, though.

You write:

I have no intention of taking TA jobs away from other potential candidates. Instead, I will only target some courses that 1. do not need TAs

which appears to satisfy #1, though I think some would be reasonably concerned about devaluing TA labor more generally - this is not something easily measured.

I think you'll have to make a determination on #2, it seems, though, that you do see some benefit, so you'll have to weigh that against the costs.

My recommendation would be to approach this not as looking for a volunteer teaching position, but asking for some mentorship in developing your own teaching resources. You'll need to figure out what exactly you are asking for in this relationship; what it seems you will be providing is access to the materials you produce. It'll be up to the individual instructors to decide whether their time is worth what you provide. I'd suggest that if they spend as much time mentoring you as they would spend if they created similar materials on their own (not necessarily equal to the amount of time it takes you), that seems ethical to me. If they're going to spend almost no time mentoring and get some teaching materials for free, well, I'd suggest you're undervaluing your own time and should consider how to better balance the relationship.

I believe volunteering is ethical for both the person doing the volunteering and person/organization "receiving" that volunteer work when:

1. No actual or hypothetical paid position is affected

2. The volunteer gets a benefit equal or greater than what they put in

I believe these principles apply even if the volunteer work is for a private company or powerful university. Volunteer work for other causes can also be ethical even when these principles are violated, though.

You write:

I have no intention of taking TA jobs away from other potential candidates. Instead, I will only target some courses that 1. do not need TAs

which appears to satisfy #1, though I think some would be reasonably concerned about devaluing TA labor more generally - this is not something easily measured, and I think you want to consider the "hypothetical" job and not just the actual one.

I think you'll have to make a determination on #2, it seems, though, that you do see some benefit, so you'll have to weigh that against the costs.

My recommendation would be to approach this not as looking for a volunteer teaching position, but asking for some mentorship in developing your own teaching resources. You'll need to figure out what exactly you are asking for in this relationship; what it seems you will be providing is access to the materials you produce. It'll be up to the individual instructors to decide whether their time is worth what you provide. I'd suggest that if they spend as much time mentoring you as they would spend if they created similar materials on their own (not necessarily equal to the amount of time it takes you), that seems ethical to me. If they're going to spend almost no time mentoring and get some teaching materials for free, well, I'd suggest you're undervaluing your own time and should consider how to better balance the relationship.