Whatever you determine on battery size, keep in mind battery charging technology has heavy transfer loss. Some experts say only 70% is transferrable. So 10K mAh will be good for 7000mAh transferred, conservatively. Could be a little higher.
Garmin 5X has a 430mAh from what I have found. Can't find anything on 5XP. So I use that as my base. Then most power banks are probably 60-70% efficient, making your 10K really 6-7K usable. So that should help you...