Everything except the Y key you can get with a Perixx Periboard 535. It uses mechanical Choc switches.

The closest thing I've seen to moving keys to the other hand is keyboards with two B keys, so you can type it with either hand. I haven't seen that for the Y key.

You didn't specify whether you want row-stagger or column-stagger. Given that you don't want to change your typing habits, I'm guessing row-stagger is what you'll prefer. Transitioning to column-stagger would throw out a lot of your muscle memory and require some time to adjust, especially if you're used to non-standard fingering. Typing Y with the right hand would be a minor change in comparison.

The Y key is your breaker. That may force you to design your own as that is a pretty specific requirement not often included.

Charybdis + A Separate Num Keypad maybe?

I'm short, no, the keyboard you are describing does not exist. As others have pointed out, you might need to design your own.

Besides the Y key requirement, the curved requirement is also going to make it a little trickier to design. If you want it curved then you'll have a hard time making a PCB, so you'd probably want to hand-wire it in a 3d printed case or something. If you don't necessarily need curved and would settle for tented then it'll make that a little easier. Tented means when the two halves are tilted opposite directions with the middle highest and the edges lowest (like a classic A-frame tent). This would allow for two flat PCBs. You could make the halves separate or attached to each other depending on your preference. Presumably the numpad would then be flat relative to the desk.

The closest thing that came to mind, by the way, is the keebio KBO-5000 as this is a pretty traditional tenkeyless layout (no numpad) but split, and can thus be tented with the addition of some feet or a wedge of some kind, then you can just have an external numpad. But as you can see, the Y key is in its correct spot on the right, rather than the left where you'd prefer it. Either way, hopefully this provides some inspiration for your design.

Happy designing.

My ergo journey started with similar requirements to yours - specifically including the Y and B keys. Along the way, I learned how important layers are for comfort, ditched QWERTY entirely for Colemak DH, bought a 3D printer, and ended up at 40%. Several years ago, there was a term "1KFH" ("one key from home") people used to describe the amazing amount of comfort they found when they never had to move their fingers more than one key away from home position, nor to move their hands.

I'm not saying you have to change your requirements, now or ever, but I think people who start to make their own ergo keyboards may be subject to this sort of requirements drift, such that if they ever make it to the product phase, their products aren't what they initially expected to be building. And maybe this sort of dynamic is what makes it less likely for the product you are looking for to have been built already.

If you cant find one, making your own keyboard isn't too difficult. It's a great hobby project and there's a lot of ressources online to build one.

I guess this is called a “full keyboard”?

Used to be "104-key keyboard", or "full-sized" but I see "percent" measurements of that 104-key as being increasingly-common today.

https://www.keychron.com/blogs/news/keyboard-size-layout-buying-guide

Some of the splits like the zsa moonlander could handle your 'y' requirement, but wouldn't match the rest. With layers you can do a lot of what you're asking for though.