are you getting an animal companion or a familiar?
In terms of animal companions IMO the Giant Mantis and Dragonfly are probably your best options. Both get darkvision and while the mantis has a climb speed the dragonfly has perfect maneuverability meaning for all practical purposes it has a climb speed. Each get a bunch of nice extras as well.
The giant solifugid might be a good pick for you. It ticks the boxes you want in a companion (namely, darkvision and a climb speed). It's also fast, starts out with 3 attacks, and gets pounce and rend at level 4, which means that it's gonna be doing a lot of damage from round 1.
Also, if you google image search them, you will see that regular solifugae are super creepy. Which means that expanding them to the size of a large dog would make them even creepier. Not sure if that's a roleplaying plus for your druid, but I thought I'd throw that out there.
Animal Companion. I hadn't thought of the dragonfly, but you're right they have excellent stats. I guess my only hesitation is that neither mantises or dragonflies live underground. From a role-playing angle, it wouldn't make sense for a Duergar to have one. Not at 1st level, at least.are you getting an animal companion or a familiar?
Maybe it doesn’t happen in our world, but fantasy world ecosystems are crazy. Just say it’s a cave mantis or a cave dragonfly.