Ouch.....ok....that's going to take a while.....
The surefire way to Farm souls is in Chaos Dungeon, there are many equipment's, spells and souls there, and some juicy lore
Another Easiest way is to do Ashen Reignited boss rush, it give you a hefty amount and exchange those for Souls of Outer God from Mabel inside Chaos Dungeon, but to do this boss rush I'm pretty sure you need to beat the old king in the depth of chaos dungeon first.
But if you haven't finish the story, i recommend to at least finish one of the ending before going there, its more fun to play it the intended way, as chaos dungeon can rapidly increase your stats thereby make most of content before dlc 3 trivial
if you dont want to go chaos dungeon but want to farm soul, you can reduce your sen to 0 and go to crash chamber bonfire, there are enemies there that will drop quite a bit of souls, but they are pretty strong so make sure to gear up properly
also keep in mind Difficulty affect the amount of souls that you receive for most enemies, so you can set to higher difficulty to farm more souls