If you want to get a job inside Thailand the DTV is no good to you as you can't get a work permit on it. No work permit, no job. You would be better looking for a job before you come and arriving with a Non-B visa but you need an employer to support your application
You shouldn't be able to get a 5 year licence on a DTV but it appears some people have been successful. I know my DLT office specifies that you must be on a Non-immigrant visa. If you login to the DLT app and make an appointment for the office you want to use it should tell you
Normally if they are worried about how you will support yourself over the duration of the visa they will ask you for an explanation. Perhaps there was something else about your application. Did your application get rejected or cancelled? The latter implies there was something wrong with your biodata
The way to avoid putting money in a bank account is to get the Non-OA visa before you travel to Thailand. It will get you stamped in for 12 months but does require insurance and police check. If you you travel back to the UK every year just rinse and repeat
You can't use an investment portfolio, even cash inside an investment account is no good. You need to withdraw it and put in a personal standard bank account