@Wayne *********
yes you can apply for the Thailand Pass while in Thailand.
When you reenter you will be stamped in until the ending date of your OA insurance policy....which is December if you took out the policy last December.
No need for the added covid/health insurance policy. Your current one will meet the Thailand Pass insurance requirement.
BUT...MY RECOMMENDATION
I would go ahead and get another COVID extension...giving you added 60 days. During that time an added BORDER RUN option likely will happen at Nong Khai among several others.
That would be out and in same day...
So get the Covid Extension before April 6 ...sit back and reassess the options in coming 2-3 months.