You can’t have to active visas at the same time. A METV is valid for 180 days after it’s issued, then it’s automatically cancelled and you can apply for a DTV.
For tourist entries you formally should be able to show proof of an onward ticket within the initial 60 days you’ll get stamped in for. You could call your airline and ask for their policy regarding this or purchase a cheap onward ticket to a nearby destination.
I assume you already have a bank account as it’s impossible to open one on tourist entries. If not the best option is to apply for a Non O before you leave which also is excused for onward ticket requirement.
You don’t have to have any funds seasoned(x) for the 90 days Non O in country unless you apply at Jomtien immigration. But it’s (almost 100%) impossible to open a bank account without a long stay visa now unless you use an agent to do the whole visa process for you and still are able to open an account as a part of the total package.
(x)Extension of stay requires two months seasoning for the application.