Depends on how high you want to go, and how steep things are, and how much support you need of course.
The advantage of already living in the alps is that you tend to start out at high altitude, for instance getting to 1500m for me is only a 600m alt difference.
As you say not so light... when I take my little dude along, thus including Thule cart I got the extra ~40kg behind me on top of my own not so light weight; then taking into consideration muscle power (the more you provide, the less the bike has to do), in my case that ratio seems to be ~60% bike, 30% me depending on the path taken (and I turn to 'off' for downhill and "flat" (does not exist here really) areas), and I working hard on improving that, which means more weight for muscles but less weight because of that spare fuel called fat.
Taking those factors into account, I have gotten down to 20% battery in solely riding eMBTB mode on my Rail 9 + Thule Sport + kiddo, when doing 2x 500m altitude difference (up one hill, ride down in the middle ["weeeeeeee...." from the back
], up another hill). Which IMHO is pretty darn good for the battery. Then with getting there and back home again, is a decent few hours of riding and proper exercise that will keep my muscles nice and toasty till the day after.
As such, yes, maybe, for a much longer trip, a second battery can be helpful; I have definitely considered it already. In my case though I can just toss that in the back of the thule, won't notice much of it though.
Hence, like a EV car, driving with battery, takes a wee bit of planning and knowing how much you consume.
But unlike an EV car, with a bike if you are running out, you can always, say at 20% left, turn to ECO mode for instance, which will still support you (so you are not completely dead muscle wise). You will be slower, but you'll get forward, which can just get you over a hill.
The good thing is, at least if you run out at the top, going back down is still easy
Also, if you think you might have access to a charger during your trip, could always consider taking a charger along and taking a break. Though with biking and weight of the charger and duration of charge, I think the extra battery might be more useful.
PS: Many bikes 'peak out' at 130kg load on the bike itself, check manufacturer just in case. (bit silly btw, as when you are above those weights, an e-bike is excellent way to get you out there and to get started with exercise to lower that weight you have with you... just be warned due to extra wear on breaks/tires etc; I personally do not think it is too harmful)
