I have found that using GPS and Galileo together helps with SV coverage when you don't have access to a roof with full sky view, while at the same time not degrading the timing stability. But also not improving it.
I also found that enabling GLONASS has a major negative impact on timing stability. Weird phase jumps, overall less stable. Might be a firmware issue with the M8T, but that is what I have.
SBAS - same. However, SBAS is quite helpful during the survey-in, because it will allow the true position to converge faster. Therefore, in my own GPSDO software, I will switch on SBAS when starting a survey-in and also increase the measurement frequency to 4Hz, which is something the M8T can easily handle. Once the survey-in is complete, I switch off SBAS again and also go back to 1Hz measurement frequency.
Hope this helps.