@Zhiying *************

i sent you a PM, Please check your inbox

just ignore these answers.

1. Will getting a 30-day covid extension of visa on arrival making getting tourist and student visa later more difficult? - you need to talk that with the institution who would provide you the ED visa. it might or not, depending on the officer who approves the visa or your past in Thailand

2. When you get a student visa, can you leave Thailand for a few months and come back and still use it? Is there a time limit for time outside of Thailand before it becomes invalid? - you need to talk that with the institution who would provide you the ED visa. I haven't used it before but I know a lot of people did. I assume that if you leave and don't come back to Thailand for a long time without an excuse, it might not look good and immigration officers can deny your entry if they want.

All your questions are about ED visa so you need to talk with the institution first I assume.