@Ben *********
I got no idea how that 6 month smart visa actually works sorry. I see a trip to your immigration office in the near future to ask them.
BUT seeing as you're actually on a 6 month entry stamp from a SMART visa that you got out of thailand at a thai consulate I don't think you need to cancel anything. I think you just apply for a 60 day covid extension straight off that 6 month entry stamp you have in your passport.
Again though I don't know SMART visa stuff (few people do, they're NOT prevalent at all here). Go ask the immigration office next week before your current stamp expires.