Bluetooth uses the 2.4ghz frequency band which is free for use by manufacturers for everything from baby monitors to wireless doorbells so is prone to interference. It is not a standard adopted by IEEE and has from the outset been unstable and lacking i n standardisation. Some improvement can be achieved by staying away from sources of interference including wifi and resetting the Bluetolth connection every time ylu use it. If bt is lnly used to create the initial handshake before a wifi connection is made it at least deals with the problem of bt randomly disconnectingYou don't need to say sorry, I was just joking...
If you Google SBC-D03 or SBC-D04 (same TCU as current 2019 model but including GPS module) you will find the original manufacturer Dayton Industrial:
https://fccid.io/O4GTTHMI (contains many links and pictures of the TCU design)
https://fccid.io/O4GTTHMI/Test-Report/Test-Report-3938630
https://fccid.io/O4GTTHMI/Letter/Model-declaration-letter-3938620
https://fccid.io/pdf.php?id=3938612
I have the feeling you know much more about bluetooth standards than I do. Can you find based on the information above which connection standard the original manufacturer has choosen? Maybe this can explain the compatibility with iPhone 8 and Android devices like BV6000? Mostly it works but first connection attemt costs sometimes 3 minutes and sometimes 3 seconds. It just feels unstable. I don't understand the reason behind.