What is the entry deadline for the DTV visa if I apply now, and where should I apply from Singapore?

It’s a 5 year visa. You have until that time to enter Thailand.

Some embassies may not allow applications earlier than 3 months prior to travel date.

The visa is valid for 5 years from date of issue. You can enter Thailand any time during that time. Fir example of yiu had it approved now but did not enter Thailand until you finished up in Singapore in March that is fine. As for embassies Jakarta is getting good reports but is going eVisa after today I think it is. Go to Cambodia, Laos or Vietnam. Taipei is another option. As yiu alludecto the Thai Embassy in Singaoore is a strange one and has been for decades. It was when I worked Singapore. Getting a B Visa for a Work Permit in Thailand is a cinch though.