The train thought was really all about the previous day train for your flight out and your next day train after your late arrival, rather than substituting a very late night train to Wimbledon/very early morning train from Wimbledon.
When this question was posed the first time I didn't answer because the last time this was asked I was heavily bruised by point scoring by certain other contributors. I and lots of other people think the hotels are in walking distance. The point scoring last time was of the "don't be so stupid" type.
That is why I was really careful to check and double check on the bus option to and from the airport 24/7, and also to say that you could always jump in a cab to the hotel when you land, in the name of ease and practicality. I am also aware that you seem to be a woman travelling solo.
East Midlands Trains to St Pancras are way cheaper than Thameslink. If it was me (and it isn't) I would probably do the District/Circle line Wimbledon to Kings Cross/St Pancras (or South Western Trains Wimbledon to Vauxhall and Victoria Line Vauxhall to KC/St P) then an East Midlands Trains Advance (train specific) fare to Luton Airport (including the DART) or Luton [Town}. My own view is that if going to or from the hotel Luton Town then the bus #A is the most sensible route. But others may validly vary on that score.
That is £3 St Pancras to Wimbledon off peak and EMT Advance Fares St Pancras to Luton Airport or Luton Town are sometimes findable for as low as £3.30- although £6 to £7 is more common. Then £2 for the bus or even a cab from Luton Town station.
If you were going direct to/from the flight then the DART from the Airport Station is the way to go. Although the sidewalk is busy 24 hours a day with pedestrians it is one steep hill up from the Airport Railway Station to the Airport.
Staying at the (any) airport is the best thing for these kind of flight times. Luton is an odd airport - I know of no other where so many people walk in, or so many people sleep/sit in the terminal overnight prior to early flights.