Transitioning without T? Any tips?

You could try using natural looking make up to contour masculine facial features. There are tutorials on youtube.

 

First about your concerns:

Hair loss: There is something called regaine, you put that on your hair and it works. Also hair loss is no guarantee. Look into your family (both sides, since hair loss if coded on the X and you have two of them). Apart from that in worst case there is a surgery which german politician Christian Lindner did.

Weight gain: It may also work the other way round. There is a german FTM blogger, I think his name is Julius, who was build a little heavier and lost weight. That‘s because your metabolism speeds up, so if your calorie intake remains the same, you will lose weight.

Now to the tips:
Assuming you don‘t want and probably shouldn‘t count calories I would advise you to eat more fish. Lots of it (apart from salmon) has ridiculously few calories. Also: chicken.

For your butt: If you don‘t do it already, wear tight boxers. Has the bonus that you won‘t need to adjust your packer.

For face: Check face muscle exercises.

Voice: So the best you can do is theater/singing exercises so you can access and speak with your lower range. I don‘t know what kind of music you listen to, so pick a few singers and try to hit the notes.

 

Exercise, as you said you're planning to do. Since you can't lose weight and gain muscle at the same time, my advice would actually be to bulk up and gain some muscle first. Working on your shoulders/obliques can help balance out your hips. Working on your chest can actually make it look better when you bind (at least for those on the smaller end, don't know about folks with a larger chest). And since you're binding, working on your back can help with some resulting back issues. Please try not to bind while working out, though. Or at least use a loose or sized-up binder.

You can do a bit with your voice to bring it into a more androgynous range; find a voice trainer if you can, otherwise sing and do vocal exercises in a lower pitch than usual, and speak from your chest. Don't get too enthusiastic right off since you can hurt your voice if you're not careful. Drink warm water and stop if it hurts. Keep in mind it's a slow process, and there's only so much you can do without T. You won't hear much difference for weeks if not months. This coming from someone who worked on his voice for 8 years before finally starting T.... however if you keep at it you should start to hear some difference and gain a bit of lower range. Aim for "androgynous" range since it's very difficult to get into high "male" range pre-t. Again speaking from experience as I started with a low voice for a girl.

You've said you're dressing in male clothes, which is good, but also take a look at your wardrobe and see whether your clothes fit well and if you like to wear them. For example I started off with a bunch of button-ups. They were fine, but I didn't look that good because they were too big or small, and I didn't like to wear them for the same reason. After a few years I figured out what I really like to wear (generic striped tees, black graphic tees)-- and how to wear them (some fabrics/layers are better for hiding curves than others)-- and what my proper sizes were (hint: your best jeans size is probably bigger than what you think it is)-- which ended up helping me pass better and feel better about how I looked overall.

And that's about all you can do without T that you aren't already doing, imo. It also helps if you have a friend or two willing to correct strangers on your behalf; people are more likely to accept you as male if they see other people are doing so.