Similar hardware here:

TCL 75r615 TV and Vizio SB3651-E6 Soundbar - looks just like the model in OP except it is only ~$200 and doesn't have DD+/Atmos, just DD/DTS

First off for audio sync if you have a TCL go into the audio settings and instead of "Auto Detect" select "Auto Passthrough". That solved all my issues with audio sync from DD/DTS sources. Some other sources such as DD+, which my bar can't decode, seem to still have delay issues from Plex. Personally moving to a Shield and new audio source to take it directly and only pass video to the TV... but for ARC failing completely listen close...

I found the issue with losing ARC only occurs when the TV is turned on with my HDMI switcher plugged into another HDMI port. As long as I physically unplug the cable before I turn on the TV again the ARC remains working. If I leave the HDMI plugged in then the next time I start the TV the ARC is out and the CEC device search can't find the soundbar. The switcher has an on/off switch and power supply however those don't change the result. The issue is reliably there and gone based on if the physical HDMI cable is plugged in when the TV starts. The switcher is supposed to support CEC but I suspect it and other devices may have similar issues with CEC causing ARC to crap out even though it is a separate port.