Okay, I've played around with it for you. The FiiO Q3 works wonderfully with the FireHD - no problems, just plug and go with a standard USB-C to C cable (the one included with the Apple iPad is what I used). BTR5 works, but is really finicky. There really isn't much rhyme or reason as to why the BTR5 sometimes and sometimes doesn't. When it does connect, it does 24-bit/96kHz. I played with it, tried to figure out why sometimes it would instantly connect as a USB-DAC and why other times it would prioritize Bluetooth (even though I have it set to prioritize USB-DAC mode). It would appear as though it is more an issue with the BTR5 firmware and not the FireHD. The FireHD instantly recognizes it as a USB-DAC and will push sound to it (at least I assume so because it'll stop playing out of the external speakers) but the BTR5 will only sometimes decide that it will receive it. So, if you're planning on using the BTR5 as a USB-DAC, I would say don't - just use the lovely LDAC Bluetooth instead. The Q5s, like the Q3, works perfectly - with a catch. You cannot use a USB-C camera adapter to have the USB-C to microUSB connection. You have to have a direct cable to do it. Fortunately, I have the MC06 or CM06 (whatever its called) USB-C to micro-USB cable made by FiiO and it works well. Plug it in, and the FireHD instantly pumps audio at what I assume is 24-bit/96kHz. There was an instance where the BTR5 did connect and did display 384kHz which is awesome, but that may not be the norm...? Either way, its exporting hi-res audio to your USB-DAC. Hearing the difference between 24-bit/96kHz and 32-bit/384kHz is only really possible with extremely sensitive (and pricey) gear so I'm good with it. Now, the USB-C to microUSB cable by FiiO is only like 4 inches long which, with a tablet, is less than ideal. I don't have a USB-C to microUSB straight cable though. You can buy them, but I'm going to assume that it has to be OTG compatible or else the FireHD won't recognize the Q5s (or the other way around). When using the USB-C camera adapter (that's a USB-C male to USB-A female), the FiiO Q5s would recognize and play the signal for about 10 seconds and then randomly drop it and switch to the line input which was very odd so I'm going to write that off as a no go. BUT! It could also just be a bum cable and I'm too cheap to purchase a new one. I would assume that the USB-C camera adapters designed for Samsung phones would work. I mean, if you can run a USB-C camera adapter to a Dragonfly Red on a Samsung Galaxy then surely you can do that on the FireHD as well. I'm just not the guy to try it!
Unfortunately those are all the portable DACs that I have. It'd be awesome if any other head-fiers seeing this (who have the 10th Gen FireHD) could contribute where I can't!
Quick edit: Turning the charging feature off on the Q5s (while using the type-C camera adapter) will resolve the disconnect issue. That does require first connecting the Q5s to bluetooth, turning the charging off, then plugging it in via USB, then reconnecting to the bluetooth and turning the charging feature back on when you need to charge it. None of this is an issue if you use a regular USB-C to micro-USB OTG cable. Tried the BTR5 via camera adapter with the charging off and its just a lost cause.
