To be clear, normal disembarkation doesn’t always start as soon as self disembarkation has finished. There can be a pause, for safety reasons, whilst the porters are moving large pallets of luggage around the luggage pick up hall. During this time, nobody is allowed off the ship.
If you want to self disembark (which given your train time you will) then you cannot wander off at your leisure. You have to leave within the designated time slot (it’s at least a 30 minute window, but always early). The good news is that you don’t have to register for self disembarkation, so you could wait to see what disembarkation time you are allocated and then take a view, but if you wish to use normal disembarkation then you have to put your luggage outside your cabin as you go to bed the last night.
If you are travelling light and will be time limited, I would just self disembark as you are guaranteed not to have any delays in getting off the ship, trying to find luggage, getting through customs etc. We went on Iona in the August school holidays with 5,750 others and whilst the ship felt crowded, disembarkation was very good. One of our daughters decided to self disembark and was off the ship, collected her car and on the road by 7.30am. She was very impressed with it.