Have you got a second key?
If you have a second key, start using it and see if anything changes. The solution will be simple but determining the problem will probably take months.
Also at night, put the battery on a trickle charger for at least a week. I use a CTEK Multi US3300 per the last few files in the link below.
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Replace ****8 with
disco3 co uk and put a period in each of the two spaces.
The reason for the two suggestions above is to eliminate a whole gaggle of possibilities so that you are left with merely a few dozen maybes.
It could be just a faulty solenoid in the starter motor, ie: new starter motor.