4 trips? One to apply and one to collect or am I missing something? A third party can also collect with the correct ID. Not knocking you at all mate just trying to make it clear how easy the process is.
I understand why some people choose to use an agent for various situations but the process is so easy for passport renewal. Book appointment. Fill in some simple forms and turn up on the day.
Applied in Chiang Mai. No need for an agent. Bring all docs and any shortfall they will copy for you. Was told within 4 weeks. Sent to UK passport office same day.
You won't have a problem. Really quiet airport almost everyone is transit. Walk past the first lot of electronic daft machines where where people are lined up. There's more upstairs. Can't get lost. Can't miss your flight. Cheap too. Food's edible and you get a wine and beer. Cheers.