You can use an Xbox controller in your left hand, and you can use it wirelessly if you connect a 2nd Xbox wireless controller to your Xbox (separate from your wired XIM Apex setup) and then go into the Accessories app on your Xbox and turn on Co-Pilot for that controller. I always use a 2nd Co-Pilot controller with my Apex anyway so I can pick it up when I get into a vehicle in games like Halo. I use a regular Series X controller for this because it's lightweight. I find my Elite Series 2 controller to be too heavy to use one-handed.
I've been learning to use the XIM Nexus but I'm not comfortable with gyro aim yet, so I still switch back to mouse+Nav when I play ranked games. I have my Nexus plugged into my Apex so I can swap between the two, and I inadvertently figured out that the Nexus (held in my left hand) in combination with my mouse is a very comfy setup. The Nexus is a lightweight controller like the Xbox Series X controller so it's easy to hold one-handed, and Nexus has back buttons so you get plenty of assignable buttons to work with.
I still ultimately prefer using my Sony Nav (with a silicone grip jacket) when I'm playing with a mouse, but Nexus+mouse is definitely a capable and comfortable alternative.
These are the grip jackets I use on my Navs btw. They make the controllers much more comfortable, especially for long gaming sessions:
https://www.amazon.com/gp/product/B003ZSH86S
