Sony a73 - Problem with auto focus in low light conditions (for photo)

I recently changed Canon 5Dmarkiii to Sony a73 and also I have Fuji xt3. First I noticed big time gap using Sony a73 with flash (GodoxTT685) that was almost fixed after I turned off redeyes correction and turned 2d curtain sinhronization. But I descovered another big problem – in low light (when need to use iso 2000-3000 or more) or in backlight there is huge problem with af – it can move few seconds and still cant get focus! I noticed it in 2-3 weddings and today was doing photo of birth in hospital (low light) and was soo terrible that I cant belive. Canon 5dm3 was better, Fuji xt3 and even xt1 is much much better. Is it really so crapy and I can use my Sony only during daylight OR my camera with defect OR do am I doing smg wrong OR is it any hidden secret adjustment in menu that I need to find to fix it? Tested with both lense – Sony Fe 85 1.8 and Tamron 28-75 2.8 – same (maybe lense sony little bit better, but stil same shit actually)