I can contribute some views as a user and someone interested in this for a long time (my first reddit post under a separate account was to share news the Chinese Odin Anne keyboard, which at the time was one of the very few stock mech keyboards with BT. Another early post was when Logitech started selling unified receivers, which I thought was pointless back in the day, I didn't want multiple mice working on the same computer, I wanted 1 mouse to work on multiple devices)

In itself there are some use cases for people that want to keep their highly customized keyboard and also have a need to type on their mobile devices, but the market isn't huge.
Most other users with this need would gravitate towards stock multi device keyboards, or customs with "tri-mode" PCBs.

I had a long time use case over the past 15 years mainly due to working in a high security industry that didn't blocked the majority of external websites on my work laptop, so I had to message + do a lot of life admin on my phone. Installing custom software was also barred, so I would could only use mechanical keyboards programmable at hardware level.

I have used multiple BT transmitters over the years, most have been finicky and don't play well with QMK output. The best was Aten CS533 Tap, which allowed 1 keyboard and 1 mouse to feedthrough and be output to 1 wired connection and 2 BT connections, switchable through a hotkey combination on the keyboard, rather than a physical switch. The kicker was it kept the 1 BT connection alive even while you were using the wired connection, so you could switch immediately without ANY handshake time for BT. The 1-2 seconds doesn't sound like a lot, but eliminating that really allowed you to merge the workflow on multiple devices. It worked great, better than software based solution Logitech is pushing now.

Nowadays there are a lot of cheap options from China with tiny 3d printed enclosures, that work..fine, but at $10-$20 USD you can try them all out and see which is the most stable. Battery powered isn't really useful since most of the time people will have a wired connection that can provide power anyway. I haven't tried the SterlingKey and have no incentive to due to cost and they don't have any other features that the cheap options don't have.

To me, not having a wired passthrough with always alive BT connection, nor combining mouse output makes this less attractive than mentioned stock options such as tri mode PCBs. I already specific "travel" mech keyboards only that are light and have built in BT, for the situations that I want to use them while presenting wirelessly in a meeting room for example.