If you entered April 2021 it sounds like your visa is getting close to its expiry date. If you get back in before that date, you will be stamped in for 12 months. Take out a 12-month insurance policy to commence the day you arrive in Thailand. No need to transfer any money to Thailand
You should just be able to squeeze it in. Go to immigration on March 25 and try your luck. Worse case scenario is you'll still have up until April 19 to organise a weekender in Phnom Penh.