That’s a lot of effort unless you have the dual-battery need for other purposes (cooler etc.).
I would look into a pump with battery (like posted by paddlinpals above). That way you are more flexible and you avoid the voltage drop of the tiny wire they use. To each their own… I ended up with the manual pump and wouldn’t bother with electric pump anymore. The real hassle of an inflatable is not pumping, it is folding down and drying.
12 volt capacity won’t be an issue for me, as I’ll be using the pump on a sailboat with a very adequate house bank.
I also have an inflatable kayak. Won’t be needing the electric pump for it, as it’s super easy to inflate manually.
But I get your point about the hassle of putting it away. For what it’s worth drying and stowing the kayak is magnitudes worse than the SUP.
Also, if there’s excessive voltage drop, it’s possible the 12v power cable isn’t of a thick enough gauge.
This chart gives the proper wire gauge to use. It’s a good reference for doing any kind of 12v work.
I didn’t measure the wire size. But it was listed to have a 12A fuse. So we would need at least #14 wire and it sure didn’t look like that.
Worse, the voltage drop will cause the motor to draw more current, which increases voltage drop even more. I guess if one is willing to lose the warranty (no!), one could use a better wire.