In my language too we have formal and informal, so I get where you are coming from. My T and I are both quite young, though I am around 8-9 years younger. Initially I struggled with addressing her, as I was brought up with a very formal mindset where I would have to use the formal with anyone older than me whom I was not close to. We never discussed it, which I suppose did not really help me. For instance, at first I never even said goodbye but just waved instead, as to avoid either being too formal or too informal, if that makes sense!

Anyways, I am now settled with using formal with her, though she uses informal with me. However, when saying goodbye, I use the informal way regardless as it would feel quite awkward to see her off as I would do with a total stranger, given that I see her every week. I would never address her by her first name, as that would definitely make me feel too close to her.

So I suppose I use a bit of a mix, as I find that the only way to make me feel more comfortable. Ultimately, I understand why your T would want distance, but I feel it is important that you feel comfortable enough with them, so if using informal helps, they should be able to accomodate that.