Well, I am not a big fan of your basic premise of using the house to start and having a standby battery
to start the engine
in an emergency
or ??? But coming back to that issue later, what you have sketched works fine.
The DC + buss is no real problem as long as it is beefy enough to take the engine
starting load. Do not fuse the starting circuit. ABYC doesn't require it and for a number of reasons I don't think it is a good idea. But obviously you need to fuse everything else connected to the DC + buss.
I prefer to take the opposite approach: use the starting battery to start so that you can monitor
how it works. Have all of your charging
sources charge the house bank and charge the starting battery with an Echo Charger
or similar as you have shown. Only use the house batteries to start in an emergency
With big house banks and particularly big AGM
banks, it doesn't matter so much. But my main reason is to monitor how the starting bank starts, so you can do something about it before you have to. In your diagram it may not work when you need it.
But if you want to do it that way, test the aux battery periodically to make sure it will start.