As an experienced landlord, it is difficult to get an insight into the reality from reading your post.
On the one hand
They have moved out and you are still thinking about the deposit.
In my opinion the norm is for the deposit to be returned on moving out, even if the law allows some leeway.
Is it really that bad?
For example, the front door keys are not returned. To take the cost of replacing the locks out of the deposit
The keys are a thing of nothing.
In my eyes you are painting yourself in a poor light.
I am not a "landlord" by choice, I was just helping out an elderly relative.
This is completely irrelevant.
Actually I am struggling to find an "on the other hand" Dirty, light bulbs, not a lot.
A landlord needs to manage the moving out process just as much as the moving in process. You need to be telling them in advance what you expect from them in order to return the deposit. If you get all or most of that, you should return the deposit when they move out.
You didn't manage the process, ( I know they left early without telling you, but were you in contact with them discussing the move out, were you managing that process) and now you are in a mess.
You should do either of two things, (I suspect you will do neither).
1. Return the deposit in full now and move on.
2. Withhold the deposit in full and get started on the paperwork to justify that. Receipts for the locks, the cleaning, the skip, and all the little jobs you say really mount up. If they claim with the RTB, counterclaim for costs in excess of the deposit.
I know which I would choose.