2. Language doesn’t qualify for soft power (usually). Thai cooking, Muay Thai, or medical or dental appointments do.
3. See point 2, but you can still learn Thai on a DTV once you have it.
4. Yes the visa is valid for 5 years.
5. Start by checking out the Thai embassy/consulate that you’d use to apply. Their website will list their specific requirements. All differ in processing times. Some are a matter of days, some weeks.
Your passport. A print of the DTV and ensure you have filled out a TDAC. You almost certainly won’t need a print of the TDAC but won’t hurt to have it.