Jon Warner your situation is very easy (banking lump sum). I not see any difficulties, and apply as soon as possible (30 calendar days before your current extension expire, including last day and application day). In case of any problems you will have a plenty of time to fix them.
Map of border control hall 2 at BKK airport. As you see the re-entry booth is before border control (people movement from left to right). There is also seats where people may wait while re-entry processed.
??? I get at least 4 multi reentry permits at BKK airport, the booth is right in front of border control. so you can't technically pass through it / stamped out before get re-entry permit.
Don Mueang is weird - the people get stamped out first and pass border control then buy re-entry permit. It's like you invalidate your visa or extension first then get re-entry.
To be honest, some time ago (3 years or so?) re-entry booth was located in the main hall before security. But this is still emergency booth and most people advised to get re-entry at immigration offices.
This is great to use electronic services as much as possible to save time. The problem is, if it failed then you must travel to immigration, not submit new application. I can’t spend half working day to do this, and mailing report in much cheaper, also not failed so far in several years...