Not OP, but I think TLM works best when it's a transitional role. You have someone you want to groom into a full-time manager, and you have a team that you plan to grow over time. TLM itself is not that efficient, but can lead to strong full-time managers who understand the team really well and had time to grow into the role.

I was thinking this too. Tech lead/developer and manager are two completely different jobs. I can see TLM as a useful transitional thing, while the person is being trained or mentored at being a manager (and hopefully not just thrown into the deep end). But 6 months, max 12, I think. Otherwise it just becomes a role where someone has two jobs and ends up overworked.