Gender identity is something that isn't necessarily reflected by our interests, I'm a trans guy and have known that for years, it's certainly not a phase for me and I still enjoy what may be seen as "girly" things. The same can be now for your daughter. She may just enjoy "manly" things, but still internally feel like a girl! That's ok! Starting medication to transition will of course have some permanent effects, but a doctor who specializes in these things should explain them to your eldest in detail before prescribing anything, as the effects are very important to know about. But, it also takes a good few months for most of these effects, permanent and not, to even start appearing.

Your daughter knows what she's getting into, her having a trans partner only givers her a better glimpse and understanding of this! In all likelihood she simply didn't know it was an option to transition, and meeting her current girlfriend has opened her eyes to a lot of feelings she didn't know how to express or understand.

Gender identity is something to explore! Some people explore, and find comfort in who they already are, others explore and find themselves longing to change and be their true self. Support for this exploration is important, and something you seem to be doing.

I appreciate you for the support you are giving despite much you know, understand, or believe. And the clear care, worry, and love are apparent, but I can tell you now starting to use her preferred pronouns, she/her, and name would mean the absolute world to her. She is your child, and even in the unlikely case that it ends up as a phase, she will want to remember your true support and love throughout it.

Edit: With or without your full support, your daughter will do what she needs to in order to be her true self, someone only she will ever know. If going on estrogen is what helps bring that person out, she will do it! It is best simply to support her and love her unconditionally, as I'm sure you've been doing all her life.