Using pgytech landing gear does this shorten the battery live ?
Anything you add to your Mavic Air will shorten your battery flight time...some accessories more so than others.
The question is: Does the value added, justify the reduced flight time?
Unless you takeoff and land on a flat surface (like a table) or you hand launch and recover, I strongly urge using the landing gear extensions. Your ground clearance is practically nil, and that camera assembly is probably the most expensive single part in that drone, so you really don't want to expose it to anything more than you have to. Another advantage to having these clip on leg extensions is that if you collide with something, there's a good chance that the impact will snap one of them off, taking up some of the energy of the collision. Actually, the folding arms of the Mavics also take up some of the energy of a collision, as they collapse.
I find strobes critical to flying a Mavic Air, due to the difficulty of spotting it when it's out at any significant distance. I attach them to the underside of the drone, pointing down. If you do that without the leg extensions, your drone will be resting on the strobe.
So my basic configuration is the leg extensions, and the belly mounted strobe (Firehouse ARC V).
I very rarely use the prop guards. They add a non-trivial weight, and if you fly into trees or bushes, they become "tree hangers", so instead of chopping some thin branches and leaves and falling to the ground, it will hang up, potentially out of your reach. Flying indoors is the one case where I would use them.