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.