I had both but sent the Gustard back to Amazon. The reasons are the following. My Gustard X16 had the worst possible BT reception. The signal broke already after around 3m. I guess the BT was broken at the X16 which I recieved.
2nd reason was the display. If you use it on the desk and you can look directly at it it might be ok, but I want to use the DAC also es Preamp and control it from 3m distance. At this distance you have no chance to see anything on the X16 display.
With the SU9 both "issues" are not at all present and the display is simply said fantastic.
The X16 is more Premium as many people said. Soundwise its impossible for me to say one is better than the other. Thats why my focus was on the usability. Even the SU9 might have the 3rd harmonic issue liu found, I am not able to hear it and SMSL at least found the issue and is on the way to fix it.
I've used the Bluetooth feature of the Gustard a couple of times for Zoom calls and didn't have any problems with it, but I was sitting pretty close. Did you have the antenna connected (dumb question, but I thought I'd ask)? The antenna is pretty cheap, so that might have been the weak link, connected or not. For me, Bluetooth is not a reason to buy the Gustard X16 anyway. If I could pay a few bucks less for the same DAC without Bluetooth, I would, since the main benefit to a DAC like this is pristine sound quality through its wired digital inputs.
IIRC, the output impedance of the X16 is 300Ω, which is pretty high. I would avoid using it without a preamp unless the downstream amp's input impedance is 50kΩ or higher. I agree that the display is small, but I knew this would be the case before I bought it. Mine stays off almost all of the time.
I've not heard the SU9 yet, but a couple of friends have bought it after I pointed them at Amir's review, so perhaps I'll have a chance once COVID is sorted here. By the measurements, these two DACs should sound extremely similar, especially after careful level matching. I would not want to bet that I could tell them apart in a blind test.
But, I agree with your choice…to focus on usability for your application. Honestly, I can't see anyone going wrong with either DAC. They are tough to beat below 500 USD.