• If you only want back buttons, the Xbox Elite controller has them. For some reason, it doesn’t have other features many other controllers have, like gyro or touchpads, while being more expensive than many, even in its Core version. I think paying 50-60€ for 4 back buttons and a slightly higher quality controller is too much, and I wouldn’t go that way.
• The XIM Nexus also has 4 back buttons, gyro, and is apparently able to convert gyro movements to the right joystick, so it also works on Xbox. However, it’s even more expensive than the Xbox Elite Core, and seems to be sold out in many places.
• The 8bitdo Ultimate has only 2 back buttons but also has gyro controls, and is not overpriced. I’ve read, however, that the gyro can be flimsy. There’s also a Switch layout version, sold with an adapter.

I would love Steam to release a Steam Controller v2 with a Deck layout (although that would make for a big controller), I’m just not sure the Steam Controller v1 sold well enough on its own to justify that.