Sorry if this has been asked before, but I have a bunch of satchels from missions, is there a way to open them all at once or do I have to open them one at a time?

One at a time I'm afraid. No way around that.

no, there's no way to mass open satchels, and unfortunately likely there never will be with the way they're currently set up.
It's a simple matter that there's too many possible drops- with festival satchels each one has a few slots each with their own small drop table, but that wouldn't work for mission satchels, which are each individually an rng chance of dropping:
> gold (random amount within range)
> Potion
> Elixir
> Draught
> equipment, which consists of
2-3 rarities
3 types (relic/armor/accessory)
7 varieties (assassin/ illusionist/ myrmidon/ evoker/ disciple/ ravager/ arcanist)
which totals at 42 for satchels that can drop two rarities or 63 for the ones that can drop three rarities
> essence, totaling somewhere between 24 and 44 sorts (not sure what species actually drop from them- might not include fabled species, and might not include lunemara+kelpari, so that's somewhere between 12 and 22 species, and two essence rarities, regular and enhanced)

So each single satchel has a minimum of 70 different potential drops, so if you could like, open all satchels at once, it'd be chaos on the system (especially if you had like, 600 satchels)

I believe Krin has said that at some point the mission/mission satchel system will be completely changed, and who knows what kind of system it would be replaced with.

Until then, one just has to have another window with a youtube video or something playing as you tediously open all of them