hallo Alex,
I think it’s time to remove the starter.
It’s really not difficult ( on a 1800 cc at least) :
1/ left front of the car a bit higher ( axle stand or whatever ) to slip under the car ( depends on your size )
2/ deconnect battery
3/ deconnect wires to the starter ( 3+1)
4/ from under the car ( near the gearbox) 2 bolts to remove ( with little spanner)
5/ remove the starter (and solenoid) from under the bonnet
6/ clean everthing , you’ll be surprised how much dirt there will be
7/ remove the cover ( 1 bolt) to get access to the brushes.
On mine they were still ok : cleaning everything ( lots of ‚dust‘) was all there was to have the starter OK.
8/ check the connections between solenoid and starter
9/ as they say : reinstalling is the opposite of … , just don’t know how they can give they right torque on those bolts.
10/ reconnect everything, use some copperpaste.
Once the job is done , you’ll say you loved it ( if the car starts ) 😕
groeten, Geert.