Flat chested cis women exist and there's clothing and clothing guides made for them. I don't have any specific links, but I would recommend looking up guides and tips for flat chested women.

I highly recommend thrifting! Look for a big place they tend to have a better selection and lower prices. Feminine cut t shirts would be the first thing I would start with, they're likely to fit ok without breasts and for me personally are in the goldilocks zone of subtle but noticeable. Generally add one to your t shirt size as a rule of thumb. Sweaters and cardigans are what I'd look for next, these are definitely a bit harder to find good fitting ones in my experience, being amab and having broader shoulders and longer arms can make it a bit of a challenge. Look for scoop necks over turtle necks, but do just keep an eye out for things you can layer nicely. Women's jeans are also very nice, again personally hitting the perfect mix of subtle and noticeable. When looking, be aware of where the waist on pairs you're looking at should fall--if you hold a pair of high waisted jeans low on you when checking the fit without realizing, they'll seem long enough but really aren't. I have long legs, even for amab people, so most end up with legs a little short on me anyways. If this is the case for you too, find some socks you'd like to show off! If you're feeling bold, skirts can be nice, but I don't have much advice here, I've only found one I like. The last major piece I'd recommend is a jacket. Having an outer layer you like and want to be seen in matters, especially if you care too much what people think about you like I do. Lastly, accessories! A necklace that suits you will go nicely with your new lower neck things, and bracelets are good for accent colors.

I'm trans and have been figuring out everything the hard way, so I'm glad to spread my newfound knowledge! Love to see more cis guys wearing fem clothing!

Wow I really did not expect so many people to offer tips and advice, thank you everybody.

Good to know that female clothing fit shouldn't be too much of a problem, il keep an eye on bust etc in future. In thinking something longer and maybe a little flowy might be a good start for branching out, something like a long cardigan?

Il do some more research on thrifting, I didn't mention but I'm in the UK so it's all mainly online so not many bargains to be had from what I have found. If anybody has tips for thrifting in the UK I'm certainly all ears, jealous of those of you that have large in person thrift stores!

I'd recommend shirts. Shirts for women are often not really made to fit breasts nowadays, which is sad for women, but could be great for your case. They come in a large variety of colours and patterns, and even though often you can see they're meant for women, a shirt is still a traditional men's piece of clothing.

Exact same experience here. Some tips:

1. Unless it's tapered at the waist or contouring the thighs you can wear a lot of female clothing without looking like a crossdresser, you just need to see how it works for your body type
2. Playing into #1, shop second hand. Almost everyone who has good style does. My style sucked bad before I started buying on Vinted and Sellpy (the former doesn't let you return clothes, keep that in mind) because as you said the regular market is a boring wasteland for men. Takes more time than regular shopping but you'll find real gems. With some pieces I'm not entirely sure whether they are male or female but they look good on my build and that's what matters.
3. Look into sewing if you need a cool hobby because once you are good at it (which takes time to be fair) you can make whatever you want, however you want. Remember that all clothes you buy are made manually and in a short timeframe to save labour costs. Good beginner machine like the Brother CS10S are 150 euros and if you want industrial grade hems you just need an overlock/serger for 200 to 500 euros. Lidl has one too for 170, that I know two very skilled people have bought so it seems to be fine. Even embroidery machines are pretty affordable now, the Skitch PP1 for example.
4. Use Pinterest to build a sense for aesthetics, that is interdisciplinary. What that means is once you understand why clothing looks beautiful you also understand why photos look good, why interfaces are appealing and art is intriguing. It all revolves around the same variables: contrast, degree of intricacy and volume. Doesn't matter whether you look at male or female pieces.

I wear women's clothes all the time, but I'm skinny, YMMV. As always, just try stuff on and see what works. Thrift stores are your friends, always loads more women's clothing.

A tiny bit of tailoring can go a long way if you are willing to learn ! I'd say to aim for big wide pieces to start, maybe outerwear ? Like a big swooshy coat if that's your thing.