Maybe you have the 80A and 250A breakers swapped? 80A is small for an engine start, and 250A is large for solar.
Using an isolator instead of a DCDC charger will overcharge and damage the start battery. Reason, you will be charging the LFP to about 14.5V, and will probably have an absorption stage at that voltage for a bit. The start battery will be subjected to that charging as will, even if it is already fully charged. It would be better to get a DCDC charger, and use that to charge the start battery.
The inverter needs a fuse. As will any loads that aren't on that drawing yet.
Understand, the SOK monitor is not for reading voltage, but to calculate and display state of charge as a percentage. Voltage is useless for determining state of charge with LFP. % SOC is easier and more friendly.
@JAGA II If you end up near Annapolis and need any components, suggest looking up PKYS. They are a marine electrical supplier. Do not think they sell SOC as they are a Victron dealer but other components may be found there.