There are no good options from Kanchanaburi. Easiest is probably getting to an airport in Bangkok and flying out and back in. Otherwise you're in for a very long drive to a border.
Then definitely go to the immigration office and ask them for their handout for a retirement extension. It's a pain to go all the way there just for that, but it will save you from making extra trips when you missed bringing something on the day you want to apply.
If you have a passport from a country that's eligible for visa exempt, then just do a border bounce with an agent. You'll meet them very bright and early in the morning, and then they will drive you to the border and make sure you get stamped out and back in with a new 60 day stamp, then drive you back. All done in the same day.
Only for Thai citizens. Other nationalities won't get to use electronic gates to enter Thailand until after the ETA system is online. The only electronic gates for foreigners are for exiting Thailand.