gauvins, your LHT with the stock 37mm wheels has a low gear of 21 gear inches with the stock 26 granny gear and the 34t cassette. If you changed the small front 26 chainring to a 24, it will bring your gearing down to 19.3, which would work very well also.
So when you do some trial runs with the more or less 20kg on the bike, you can see if you need lower.

I have toured a lot with about 20kg max, on bikes with about 21 gear inches, and for me that works generally fairly well, although having a bit lower is always appreciated sometimes, and has no downside, if you use it a few times then its appreciated, if you hardly ever use it, who cares, its still there and doesnt take away from your riding at all.

to change the 26 to a 24 is fairly inexpensive, if you have the simple tool to remove the crankset, and some allen keys, then its only the cost of a 24t chainring that fits the bolt pattern of your crank (which is a pretty standard size)

If you don't have the experience or the tools, a bike store would charge a small amount to make the change, would take 15mins. tops.

I'm glad to hear you are now thinking of the EV6, it sounds like a good family route, and not just for being less hilly, I'm thinking specifically of the shorter distances between villages, campgrounds etc etc.