So I read the manuals for the products I choose. I tried to keep it economic as I don’t need much and also no need of a multiplus mainly because I don’t have much space.
My main questions are:
Should/can I connect the negative of the starter battery to the negative busbar of the loads or keeping the starter side and house battery sides separate?
Should I connect all chargers directly connected to the battery or to the System + of the BMS?
I am worried that you really do not appear to know what you are doing. I suggest that you consult a vehicle / marine electrician with Victron experience and/or read the manuals again and understand.
Yes, the system negatives need to be combined so there is a single common reference. You have also forgotten to show the negative connection from the bus bar to the BMS with the fuse in it.
Direct to the system + so that the BMS stops the charging sources if the batteries have a problem.
The battery is a source of large current so you need to protect all the positive cables with fuses from over current from the battery as they will be rated for lower current than your 200A master fuse. very cable connected to the positive bus bar that has a rating below your master fuse rating needs its own fuse and I mean EVERY connection. This is basic wiring know how and is why I question your knowledge. Search for the book Victron Wiring Unlimited and read this, it will provide much basic knowledge.
What is the Orion 12/12-18 there fore, I can not see any reason for this as it is duplicating the alternator charging because it is in parallel. Do you know if your alternator outputs a high enough voltage to charge to batteries, it needs to output at least 14.4V, 14.5V would be better.
Consult a vehicle / marine electrician with Victron experience.
