If the money you bring onshore has already been taxed under a DTA, you do not have a tax liability in Thailand even though you are technically a tax resident.
The property owner is responsible to file a TM30 within 24 hours of an alien arriving. You only need to worry about the TM 30 if you are going to immigration for an extension
visa-exempt entries are for tourists so if you are here to visit your son you need to get Non O visa based on visiting family for more than 60 days. You will get a 90 day entry when you arrive
My friend did it last week and it took 5 working days. If you're not sure ask on here or PM me. I've done 3 in 3 years due to circumstances beyond my control.