Danny

Danny,I cannot  give you an answer on all the questions but I'll give it a try:
1). Read @Youda's lab from here. He moved to pylons as well and later on he expanded. Very nice to read!
2). Does your Victron have MPPT inside? What type is it?
How much power are you drawing from your system?
Rik
 

Hi Danny
Do you have the 8kVA Quatrro?
For Victron, you will have an MPPT charge controller that you connect your solar to. I’m pretty sure you can add as many as you like.
The 8kVA is just the maximum amount you can invert. So basically, take from DC to AC (or AC to DC). But on the DC side, you should be able to happily charge as much as you want.
As an example, assume you have the momentary potential to generate 10kW with your PV. Your house’ demand at that moment 6kW and your batteries are 50% full. Then of the 10kW, 6kW will be inverted and used in the house and the remainder would charge your batteries.
If your batteries are already full, you would only invert the 6 and the other 4 would be “wasted”.
Regarding adding additional batteries: My advice (for what it is worth) would be definitely to add more, like you intend. The 2xUS3000 is technically under spec for just a 5kVA inverter. For your 8kVA inverter, they will struggle to meet even just half the demand when you are getting close to your 8kVA inverting capacity.