"In other words, any downside to proper functioning to have extra cartridges that aren’t always hooked up to the battery?"
Nope! No issue to having multiple cartridges and swapping in and out. Always make sure to store in a cool place however.
This is exactly what you SHOULD do! Throughout the day you might want different cannabinoid ratios to accommodate different activities and different medical needs (pain, anxiety, insomnia). You can carry around your pen and your different cartridges, as long as you’re reasonably careful. I keep the packaging for each cartridge so I can carry them around safely.