Doing this splits the battery bank into 2 different banks, which is precisely what you do not want.The one exception is that the middle battery and first battery positive jumpers which both terminate on the Class T fuse block rather than the middle battery positive jumper going to the battery post of the first battery.
The Class T fuse must be within 7 inches of the battery terminal of the last battery in the parallel bank.
First is 11 inches longer than the ABYC standards suggest.Are you saying that the 18" of 2/0 AWG between the first battery post and the Class T fuse is electrically significant?
If all of the jumpers are 18 inches there is no problem. If some are 6, some 18, and 48 inches then there will be problems with maintaining even charges across all batteries.
The problem is not the absolute voltage drop, the problem the batteries will see different voltages when charging and when discharging the first battery will drain faster than the last, leaving the batteries unevenly charged.As far as I can tell, the only difference between the ideal setup and what I have is that the positive jumper from the middle battery to the first battery terminates on the Class T fuse, rather than on the positive post of the first battery itself. AFAICT it all boils down to 'is 18" of 2/0 AWG significant enough to make any real-world difference'?
For a 300A load at 13.5V:
Voltage drop: 0.091
Voltage drop percentage: 0.67%
Voltage at the end: 13.409
I'm not certain when ABYC adopted the 7 inch standard. If you continue with the factory wiring, any problems would be grandfathered. Since you are going off on your with your own design, any problems that develop are on your shoulders. A good surveyor will note the problems, which may affect any future sale and insurability. IEDIT: My guesstimate at 18" is from memory, and thinking about the ABYC standards and the ISO/RCD standards of 7", it could be less than 18" if Hunter tried to be compliant in 2007.
Your boat, your risk.