As far as I can make out, you haven’t swapped the serial port mapping with an overlay either. A couple of points on that (although I’m now thinking this should probably be discussed in the emonScripts thread?), firstly the overlay currently used on the emonSD disables BT rather than swapping the serial ports around to keep BT functionality, however as far as I can make out, by enabling the serial port with uart_enable=1 in config.txt and using the default serial mapping (ie just disable the serial console, not the serial port and do not use a custom overlay) then the clock speed is reduced and fixed at 250kHz to stabilise the serial port, effectively under-clocking the processor slightly, but more importantly disabling the “turbo” type boosts. So rather than swap the serial ports around and underclocking as well, if we just use the ports as originally mapped (and disable serial console as we must already) it should be simpler (and run cooler?) without losing BT.
There was a new release of Buster Raspbian on the 10th July. So I hope we’re all using that now?