you don’t need an onward flight with a DTV but if you get a extension on a non-o it’s only valid as long as your passport is,so if you have 8 months that’s all you get
Never heard of it taking a month usually no more than 10 days and that’s a lot,people do come back to Thailand to wait but if your asked for more documentation and proof of your location you’ll be denied there are agents who will help at crazy prices 100,000+ up to you