[Misdirection]

Is the cable from the batteries to the port motor longer (further away) than the starboard motor is? If so, you could try adding a second similar sized wire to run in parrarel to help carry the load.

Have you measured voltage at the batteries vs at the starter while cranking looking for voltage loss?

Have you cleaned every connection (and added dielectric grease)?

You could swap starters to see if the problem follows it. That might be a way to test the starter.

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