Thru axle impossible to remove with multitool

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.

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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.

I heavily recommend not carrying a multitool, but rather a set of shortish Allen and Torx keys that are the exact sizes that you need for your bike.

I have weighed a number of multitools and found that usually they are not only more cumbersome to use than individual Allen/Torx keys, but also often surprisingly heavy as well. I don't believe I have any weight penalty by choosing to carry individual keys. Of course carrying the longest keys I can find would have a weight penalty, but I choose to carry short keys instead.

The only "multitool" I carry (if it can be called as such) is a chain breaker that happens to have cut-outs that can be used as a spoke wrench. The chain breaker is operated by an external Allen key.

Here's a list of what I carry. My 6mm Allen key only weighs 29 grams. The full set of Allen keys (including 8mm which you may not need, 56 grams), two Torx keys, a Philips/slotted wrench and chaintool-spokewrench weighs 220 grams. The weight of a typical multitool having the same is around 200 grams, and doesn't give you enough leverage on the largest sized Allen keys as you have found. Plus, with a multitool you are forced to use what the multitool manufacturer has decided to give you, whereas with individual tools you choose what you carry. One example of a multitool (Crankbrothers M20) weighs 203 grams and lacks T30 Torx key (17 grams to carry the T30 key separately), so not really a single gram saved.

Lack of torque/leverage when operating a multitool in field conditions is typical for the situation when loosening/tightening the pedals, not the wheels.

Ironically, an extracted through-axle can be used as such a lever when attached to the multitool:

Image source

In your situation, it is only applicable if you've managed to unscrew at least one of the through axles. It does not have to come from the same bicycle if you are traveling with someone else.

In general, your travel toolbox should suite your requirements, as no multitool is truly universal. Some multitools are just larger than others, so getting a bigger one is one way.

Adding a separate long 6 mm key to your toolbox is the easiest solution, I think. I do carry such a key when I know in advance that I'll be taking the pedals off and on.

If you have nowhere to put it, tape it to your pump, or to the frame. You do carry a pump with you, right?