Yes, know what you mean, can easily get stuck on some one detail for days.
Don’t think so, since its working for you now and the serial link is a basic function.
I have a RFD900X unit but I understand the manual is the same as for your RFD868X. My firmware version is 2.53, I think.
Manual say the default settings for Serial connections are:
57600 baud rate • No parity • 8 data bits • 1 stop bit
So I use that for setting for the BT unit as well as the PC/Phone/Tablet QGC settings as well. Its got to be the same across the link.
RFD-TX wired to BT-RX
RFD-RX wired to BT-TX
Now, here we are moving in to deep waters.
I think the reason you are asking is because you have a BT module HC-05 which can also act as a Master. That unit has 6 pins. I really never got success with these units. You need to press and hold the thing marked EN (or short them if no key is present on unit) on most units but not all, while booting up to make it enter in to config mode. I never made that work.
Its more easy to use the HC-06 unit with 4 pins that can only act as a slave. Then you don’t need to worry about that.
I think you don’t have to but if it doesn’t work then you may need to research.
Here is a version number that works with the BT unit configurator I use, BT_CONFIG.EXE, seems to be 2019.09.01
You can get it free on the internet, but scary download sites are around. If you need I can upload it to you.
Also, this is the connection you need to do from your Win10 machine to the BT unit to configure it with a Name, Password and communication speed (57600).
No, all these units work out of the box: Win10, MacBook Pro, Android for Samsung phone, and for Android Samsung tablet. For Raspberry Pi 3 and 4 it works fine after installing a Serial package sw. All connect as a breeze, most times. The most failing to connect is with the Win10 - those guys never get it right. Need to reboot etc etc.
Hope it helps
you can get the BT unit in to the same box as the RFD unit if you squeeze it a bit.