charging in back country

Battery life depends on the mode (tracking mode, activity recording) and conditions (open sky, in trees etc) and will vary from a few days to a couple of weeks (at least according to the mini 2 manual) so first thing would be to do a worst case estimate of how many times you might need to charge the mini based on conditions and how many hours a day you may use it. Worst case involves both the conditions and if you get into trouble and need to do a bunch of messaging.  Given the battery is only a 1250 mAh it would not take a large power block to recharge the mini a few times and this would be the simple solution.  Charging directly from a solar panel has risks if you end up with days of rain and in heavy wood (and the combined panel/battery systems are not that effective.  My general method is to use my smallish panel to recharge a small power pack so I can then use this when needed or go out with just the power pack for shorter trips.  A lot of this depends on if you need to charge anything else as well (watch, phone) so you need to consider a charging "infrastructure".

In the end, I suspect a properly sized battery pack would be the best solution if you are only dealing with the Mini.  A 10000 mAh pack weighs very little and would give you a number of recharges.

my Mini does some 70h with 10min transmit interval.

I have small 6000 mAh pack in addition to it.

Solar tested, not any use unless somewhere with strong sunshine. But when travelling, you might be stationary only during the night and then this will not function.

I have tested some of the kinetic devices, none of them is able to charge anything at all in reality, it happens only in the advertising videos.