1st. Bus ticket is cheaper than air tickets. 2nd there is no delay or cancel with bus. 3rd not everyone is living at Bangkok. It might require multi trip to the airport.
Bangkok bank next to Rajabhat University. Opened account with covid extension before. Paid 500baht for annual insurance. Need TM30 and a Thai friend go with you. Just make things easier. I heard someone open account with KBank at Big C Hang Dong too. Good Luck.
This photo is from Chiang Mai immigration office level 2 where you apply for resident certificate. Seems you don’t need long term visa for it. And TM30 office is at back of the main building. Next to photo copy.
My father went to Thai embassy in Shanghai for a TR visa before covid time. They ask for a lot of documents, including invitation letter and inviter house paper or land paper. There is visa agent at embassy. They ask quite a bit of money for a TR visa. ¥1000rmb up. So just visa exempt plus 30 days visa extension total of 60 days at the end. BTW he holds an Australian passport.
Base on your questions. Firstly, not recommended to travel with a 2 yrs old baby at this time. Second there are limited hotels to choose for quarantine, and needs to be on the list of those qualify hotel. Hotel fees will be around 50k baht depends on the hotel.