Hi Rick.
Yep, you explained your issue dead on, you are correct, due to that you only tried to “smoothen” the busbar to the battery you get these high inrush voltages as this busbar from the battery to the inverter is always “full” or resistances (very small for each single connection).
Therefore its always better to “start” with capacitors at the inverter itself and at the “entrance” of the busbar extra capacitors.
Yes, supercaps pricing went up 20% since this year.
The supercapacitor or ultracapacitor is opposite mounted, meaning, this one you put bang on the battery terminals, for extra “boost” from the battery.
In your case I would use 3 of these Licap 18V 62F Ultracapacitors (SM0062-018-P-1) in series, (we will mount these this year at another installation), with 2 200A fuses directly in between and then 2 silver cables to the terminals to connect them directly instead through fuse holders and cables (a lot less resistance again).
As you can see, these 3 can then be put right besides each other and linked.
Jeroen.