It's the RS232 port for programming the Rinehart inverter. Keen to do any tuning from the comfort of my driver's seat, and not under the hood.
What's the D-sub port for?

Smart to solder the Ni tabs instead of spot welding them to the 0.8mm thick Cu busbar (expensive /powerful welder needed)! What thickness is the Ni tabs? What spot welder you using?These little 1 mm wide, 8 mm long slots will make soldering the copper-nickel tabs a whole lot easier, and stronger.
You didn't show the Ni-Cu tab sandwich in your video, which got me confused. Are the tabs also cut with a fiber laser? What power level is needed on your Sunkko 709 to weld the 0.15mm Ni/ 0.07mm Cu tab sandwich to the cells?The 0.07 mm copper is on the nickel-copper tab sandwich. The copper busplate is 0.8 mm thick.
I wish. Unfortunately I have the high tech approach of rolling it out 1 m at a time, sticking the adhesive copper tape on, and cutting it into 25 mm long sections with my snips (and cutting a tiny corner off each one of these, as the tolerance on the busplate slots was too tight!)You didn't show the Ni-Cu tab sandwich in your video, which got me confused. Are the tabs also cut with a fiber laser? What power level is needed on your Sunkko 709 to weld the 0.15mm Ni/ 0.07mm Cu tab sandwich to the cells?