Wander through some graveyards, old battlefields, and other places related to death to get big. Eventually I should permanently evolve into some sort of Writhing Soulmass, Spectral Amalgamation, or maybe become the central figure of a Ghostship. Somethin' like that. Then I start playing around cutting away chunks imbued with portions of my will and try to become an undead dungeon sans core. I imagine fresh souls harvested by my own minions will be much tastier!
My first act probably be trying to get stronger, so I won't become someone else's free experience, then I can go out to steal people's front door keys when they come from outside, and they really need to use the bathroom.
Maybe I will kidnap newborn royals and put them in dungeons of other kingdoms, so war will start, and I can eat the souls of the death.
If I get isekai'd into a cultivator world I'm pretty sure I'll grow overpowered by just standing in place because of all the young masters that kill people and their 9 generations for looking at them the wrong way.
Absorb a bunch lingering souls than move onto undead souls and than human until I burst from over eating or I get bored whichever come first. If souls taste good or change depending on the soul I might just never get bored of them.