What do I need to look out for when replacing rear thru axle?

Unfortunately, thru axles are not as standardized as quick release skewers. So there are a number of basic measurements that vary by bike. Drawing from the Robert Axle Project, they are

1. Axle diameter and hub length. For drop bar bikes, this is usually 12mm diameter x100mm width in front and 12mm diameter x 142mm width in the rear, but exceptions exist.
2. The actual length of the axle shaft. Confusingly, this does vary from bike to bike, even though most drop bar bikes conform to the nominal dimensions above and most current MTBs are on Boost spacing!
3. Thread pitch, or distance between the threads on the threaded section. They can be 1.0, 1.5, or 1.75 pitch.
4. Length of the threaded section.
5. Shape of the axle washer on the non-threaded side - I think they're usually flat, but conical ones exist.

And now, the problem: you also specified that you want a thru axle with a lever. These are more rare. I know that DT Swiss is one company that makes thru axles with levers, but they only have some specific dimensions. They may not fit your bike. You could check the dimensions of your axle, or you could call Cube or a local dealer if there are any in your area.

The objection to needing to carry an Allen key with you does not seem well-founded. It's usually recommended to have a mini-tool plus tire levers. Saddle bags are designed to fit a tube, a mini-tool, and tire levers; smaller saddle bags might require a small mini-tool, but carrying a mini-tool won't preclude you from having a pair of tire levers. Thus, I don't understand the objection that you need to have a wrench, but that this is not optimal when you get a flat.