They added the new visa types in the Royal Gazette but the underlying law governing work authorization was not changed. The relevant law is the Alien Working Act, B.E. 2551 (2008).
Sticking to the letter of the law it's not legal to work remotely with a DTV either. The Ministry of Foreign Affairs introduced this visa and said people could do remote work but the no laws were actually changed to allow this.
Buying is no problem. Renting it out is technically illegal if you are managing the rental yourself because that is considered 'working'. But you are very unlikely to have problems, especially with only 1 property. If you wanted to be 100% legal you would have to use a property manager or a Thai person to manage it for you. You also need to file a tax return for your rental income, even if you stay in Thailand less than 180 days in the tax year.
I don't think the agents in Thailand that help foreigners open bank accounts operate in the way you describe. They just introduce you to a friendly bank manager that will overlook the bank policy of not opening bank accounts for DTV, tourist visa, etc. You may find it extremely difficult to open a bank account without using such an agent if you don't have a non immigrant, elite, or LTR visa.
I mean it's not illegal to use an agent to assist you with the account opening process. Of course you would still need to go in person to open the account.