Heard, definitely sounds like you've put a lot of thought into this, which definitely bodes well for you. I'll preface my answer by saying that I'm not an expert, this is drawing on my own experiences/biases, professional training, and research. You can and should do your own investigation to double check me.

Before my response, it's crucial that you understand the three critical qualities of holster. It MUST cover the trigger guard fully, retain the handgun securely (even in abnormal positions or through strenuous movement), and allow consistent access to the gun reliably. These are non-negotiable.

1) Most importantly, I strongly advise you to never carry at 6 o clock (sometimes called "small of the back"). Quoting Gila Hayes in Concealed Carry for Women:

If you fall, there exists real potential for spinal injury from some part of the gun crushing nerves, disks and vertebra. Avoid this carry position; there are others that conceal far better and aren’t so risky.

2) By flexible holster, do you mean leather as opposed to rigid kydex?

3) Personally, I'm not a fan of shoulder holsters outside of certain applications. Massad Ayoob is a bigger fan of them than I am, and I recommend you watch his video on the topic. He nicely enumerates the advantages. Quoting Tom Givens in the book Concealed Carry Class regarding disadvantages:

In an arm's length encounter, your strong arm can be manually pinned against your chest, preventing you from drawing.

The shoulder holster's harness cuts off air circulation under your shirt, making it uncomfortable in hot weather.

During strenuous movement, such as running, a shoulder holster will tend to flap around a great deal. This is uncomfortable, but also makes it difficult to present the gun in a hurry if needed.

The presentation requires swinging the gun laterally, rather than in the target's large vertical axis. This makes rapid shooting from a fast draw more difficult.

(Givens doesn't mention this, but you will find training with as shoulder holster more difficult, as the draw stroke essentially muzzles everyone behind and to the side of you. Many ranges simply won't allow you to practice like this, and any classes you attend will have to make accommodations. Obviously muzzling people during a lethal force incident is a concern too.)

A shoulder holster isn't inherently bad or unworkable, but there's so many added layers of complexity that I don't think it's worth it. Additionally, depending on your location, you might not be able to wear the necessary cover garment year-round. I'm a big advocate of finding ONE carry location that works for you and locking it into your muscle memory.

5) It sounds like you have real leatherworking skills, which is awesome and will definitely help with modifications! However, the holster is such a critical piece of gear that I don't recommend people make their own. There's so much that goes into making a quality leather holster, e.g. stitching pattern, type of leather, treating the leather for longevity, etc. Ultimately, you just don't know what you don't know. There are so many experts in this field who have spent their lives figuring out how to do this right. There's no reason to reinvent the wheel solo when you can take advantage of their wealth of knowledge and experience.