I went in to the Patong office and did this. 300bht for passport photo and all the photocopies- it saved me running around town, thought it was decent price.
Looks like a High school Maths problem. Don't quote me on this but it looks like if you get it after 3 or 4pm in Medellin on the 20th you should be OK. The problem will be getting the results quickly so you'll need to get a 3 hour (or fast test) result turn-around, if they even have that there. The US and Singapore are the ones to figure out. I would want to get the results in my hand the night before you leave Medellin.
It is hit/miss I got the Covid ext @ CW BKK 2 weeks ago. I went in 5 days early but they added it to the end of my existing Extension not from the day I went in.
A lot of Folks used Royal Vacation as a one stop shop. They had a very reliable reputation. I used them back in August for Sandbox. They have 7 nite packages on their site now starting from 18k bht.
You can go any day. For the visa extension you don't need to wait until the end of your first tenure. You can go tomorrow and 30 days will be added to the end of your initial 30 days.