Hi appreciate your sincere sharing. I'm not from SCDF but the fact that they invited you for the assessment means something. 31 isnt too old imho. I'm sure there are even older mid-career switchers who are considering signing on for $$ especially in this covid pandemic. Follow your heart, I say grab this opportunity and go for it! Job satisfaction is very important.

'Find a job you enjoy doing, and you will never have to work a day in your life.'