Anthony489, as far as I am concerned, your input to this forum is always appreciated, and worthy of respect.
It is many years since I had almost identical problems with my Talbot Express, to which I had added an aftermarket alarm, and a hidden key operated switch. Both of these security devices were incorporated into the starter solenoid circuit.
It was several years after installing the modifications, when the starting problem first arose.
I recall removing the bolted link between the solenoid and the motor windings. With the solenoid operated, I checked voltages at all appropriate accessible points (more in those days), and failed to find a significant single voltage drop. I concluded that the operation of the solenoid was marginal, and exacerbated by my added wiring. On that vehicle the starter was mounted on the front of the engine above the turbo. I estimated that with an engine temperature of perhaps 88C, it was possible for the starter to temperature to reach 100C, thus increasing the solenoid resistance by about 30% compared with ambient. May I politely remind you, that while magnetic flux density is proportional to current, magnetic attraction is proportional to the square of the flux density.
Subsequent to my modification, identical problems were mentioned in the technical section of the MMM magazine. It is my feeling, that such mentions represent only a small fraction of the total occurences.
That being said starter motor design has changed in the last 25 years, and so has the price.
I do agree that all appropriate electrical connections should be checked.
These include main battery connections, and the engine block earth strap, which I think may be similar to that on the x250. Another possible cause could be the ignition switch starting contact. If there is any possibility of water ingress to the engine bay fusebox it could have caused an increase in resistance in the starter solenoid connections.
There is an informative thread about the engine bay fusebox, it refers to a Fiat Stilo, but the fusebox is similar.
https://www.fiatforum.com/stilo-guides/313986-how-clean-dry-out-fuse-box-near-battery.html