Should I apply for my 30-day visa extension before my 45-day stamp expires on February 20, or can I wait until the last day?

You can go anytime before the 20th, and I would advise to go sooner rather than later.

Also, if you’re going to Jomtien or Si Racha immigration you will need to have some proof of where you are staying. A letter from the hotel or rental agreement. It’s not a requirement in Bangkok or Hua Hin, or it wasn’t a couple of years ago.

Dont leave it until the last day.... go as soon as you can.... as others have said they add it on to your current one.

You can get the 60 day SETV then with a border bounce that will turn into 105 days…you will need ฿1,900 baht and apply outside of Thailand for the visa

Should now

You can go for it now. Some places have different rules etc, but take your 1900 baht, take your form, passport and go now. Plus, it gives you time in case you forget something etc.

There are no options other than 30 days (other than those you mention for long term extensions).

There is no downside to going early as they add on the days to your current stamp.

You can leave it to the the last day if you so choose, but that might be risky is as much as you be unwell that day or have a tire puncture or something silly like that.

You have no reason to wait. Fridays and Mondays are the busiest days at immigration so you're basically stuck going on one of those days now, you really should have gone earlier because if anything unforseen comes up it's easy for you to go into overstay. A tourist extension is added to the end of your current stay.

You can get one tourist extension for 30 days, there are no other extensions available to you unless you have Thai family.