If the marking says 20 newton metres, then that's what the manufacturer has resolved as the correct torque. If you're looking for permission to undertighten your through-axle, then that's your personal decision.
Instead, consider getting a single longer 6mm hex tool and stowing that with your spare tube on the bike. If weight is an issue, trim off unnecessary bits, or look for a titanium tool.
There also might be multitools that provide more leverage, by folding out tools at the opposite end of the 6mm bit to give you more length thus leverage.
Lastly, adding a short length of pipe as a cheater bar to your toolkit could also provide that leverage. Light aluminium or even plastic PVC pipe can be sufficient, and it probably doesn't need to be more than 15~20 cm long.
I suggest you get/borrow a torque wrench and tighten this bolt to 20 Nm. Then use your hand tool to make sure it can be undone and done up again. You're trying to get a muscle-memory for how tight the bolt has to be.
You might be surprised how much or how little effort it takes - getting a "well calibrated elbow" is a useful skill.