Starter motor on the bench is under no load so it draws a lot less current.
If it is a bad connection you will get the click, click, but it won’t kill the battery. The battery voltage will drop under starting but should recover straight away after you stop cranking or trying to start. If it doesn’t then it sounds like battery.
I have a meter that measures the battery and does a cranking test, it tells you how many amps the battery can give and also measures the voltage under cranking (I think it s up to 800A, but you program the battery rating in). In addition it will also check the alternator for charge and ripple. They are about £30 £40 but well worth it, especially if you have more than one car, mine is an Ancel with an LCD graphic display but there are plenty of them available.
2020 blue SZ5 (one of the last to be registered in the UK)
Ex 2011 Blue Jimny SZ4
Northumberland Jimny Blog
Have you checked the fusable links?
The green wires at the battery connectors may melt just enough to make a voltage drop over them, but not enough to show when there is no load with a voltmeter. wiggle and pull on them to see if they have a weak connection. one of them is power to the ignition circuit.
They can be foreshortened a little bit and reconnected if they are of similar length to original and the break is at the far end of them. I have crimped on a new connector sometimes.
If broken it should still work when you shorten the connectors at the starter or make a leap wire from battery to the small solenoid connector though..
Still thinking of a weak magnetic coil in the solenoid or weak main coil in the starter motor. or the starter motor graphite brushes.
