@JonL , the way your batteries are wired together is probably less than optimal. Go to the link below, from the resources section of the forum. Go to section "3. Battery bank wiring". It will show you better ways to wire up multiple series/parallel batteries.
Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download
diysolarforum.com
The recommendation from Battleborn for the 300 amp fuse came from 6000 (surge) watts / 24v * 1.15 = 287 amps and they rounded up to 300. That's a bit high in my opinion.
For the battery fuse, the general recommendation here on the forum is to use a Class T fuse, not ANL.