[hiccup]

You need a qualified marine electrician to take a look. The battery cables and connections should be inspected--this includes both + and -. The performance of these cables and connections should be quantified with the use of a meter.

If all cables and connections have been inspected and performance quantified and deemed acceptable, the starter should be removed and brought to a DC electrical shop that rebuilds starters & alternators. They are more plentiful than you might think. There, they can properly test the bendix & starter and repair as necessary.

There is no reason in the world a problem like this should be challenging to solve. Not beating you up, just pointing out that this is a very simple troubleshooting and repair exercise for any qualified tech.