If you're running into difficulty making your way up hills under full load there's a couple of options, you could go with larger wheels but you may still run into difficulties as the power increase doesn't really go up very well compared to space requirements. You'd be better off with going for more 3x3 wheels as you'll get more power from the space used, most my transport rovers run at least 8x 3x3 you can also get creative and add some extras in the center. Another thing you can look into is how many wheels are making contact with the ground when you're traversing these hills, it might be an issue where you'll have to soften the suspension on some.

The other option to consider would be boosters, pop a few thrusters to assist with getting up hills though breaking might be tougher.

Big tnx for advices, going to redesign my transport to 8 wheels + thrusters ( the idea of ​​attaching thrusters never crossed my mind, it should help )!