I contacted the electrician who installed my battery bank. He said he'd charge me $90 for an hour of labor to come out and check out wiring, and diagnose the issue.
I'm not sure if it's worth it to buy a $90 clamp voltmeter which I'd use on board again and again and continue to try and do this myself, or just pay him and be done with it.
In your other thread, you are asking about knowing your limits. In my view, this is a straightforward project with little downside to seeing through yourself. Sure you can pay for someone to do it, and if you prefer that then that's ok. If you want to expand your scope of limits and understanding this is a great project to do it with.
To understand and troubleshoot electrical will take some bit of study, a good book like Calder's, read through it. Buy a good quality meter (A Fluke 101 for $60 comes to mind), and then throw yourself on the project. Without a good meter, you'll have more headaches than success, you can't see electricity (usually) and you have to understand, trust and rely on your tools to do the seeing for you. Most of the projects on your list were electrical in nature, without some study and some basic tools you can't do those things.
You don't need a clamp meter to do this job. They are handy but not necessary, few people have DC capable clamp meters in their tool bag.
You need a methodical, logical approach; start at the windlass, and test the system with your meter from the motor terminals, then the relay, then the fuse/breaker, then the battery itself.
You can get coaching and suggestions here, but you've got to put in the work to learn the basics, know what the terms mean, what the basic tests are, and how to do them. That's all book knowledge, the windlass project can turn that book knowledge into practical experience.
