Planet use Mediatek SoCs and deliver bad support. Slow security updates, if any, and short amount of software support (2 years or so?). All that for a premium price. Other companies like Fairphone do much better.
But I believe (unfortunately!) that a hardware keyboard is too much a niche. Not enough people care about it (which also perhaps says something about the quality of OSKs nowadays). Therefore a solution like this might be the next best thing if you want it attached to the device, I agree.
The alternative is to use a Bluetooth keyboard or a USB-C adapter. I wonder how many people are still using that, and if for them this would be a better alternative. Consider for example the Logitech K480. You can put the smartphone in it to stabilize it. Good enough for typing in the train, for example, but not good enough to use while typing on the street. What I mean is that your solution is excellent for usage on the go. That’s part of the specific (sub-)niche.