My experience couldn't be more different from yours, Bluetooth has been flawless, and I have been using Bluetooth extensively for years with many devices so I am very experienced with Bluetooth. Did the knob fall off? I can't imagine why people would even use the knob except for once in a blue moon. Set it and forget it, and use the remote control. Clearly some compromises have to be made to keep prices reasonable. I get that build quality is important, but ultimately it is secondary as compared with sound quality and connectivity features. This is a DAC, really no need to fiddle with it. Set the volume, set the filter that you want and once and a while adjust the input. It isn't a toy to play with so not sure how robust the knob needs to be. Yes, if you need to adjust the volume because you have no preamp, fine, but even then, use the remote.
The Topping gear is really pretty much the same thing in terms of quality, generally speaking. It isn't like Topping is a luxury brand, both are very similar. I would also suggest that the price difference with the D90 has been invested into better build, but again, to me a DAC really is barely touched after initial setup so if I have to decide between paying hundreds more or not fiddling with things as much, I tend to feel that the money in my pocket is better for my needs. Your needs are of course yours. I am sure the D90 will perform well for you, based on what I have read it is a nice piece of kit.