I've managed to beat all the challenges individually (did you know poison pursuit causes the second phase of the rat's intro cutscene to poison you for 3 unavoidable damage =]]]]] )
Other than all the unlocks using credits in the breach is there some obscure unlocks I need to be made aware of?
Does becoming the resourceful rat unlock anything?

Man i wish there was a rat past
I hope we get that in the sequel
With i think like 5000 hegemony credits you can give tonic a gold coat but there's no achievement tied to it
threre are a couple of miscellaneous hoops to jump through, don't remember them all since it's been long but the last item I unlocked, the Bloody 9mm requires 20 uses of the Lament Configurum, pretty sure that there are a couple items like this
There are some milestones that the game tracks but doesn't tell you about, like dropping enemies into pits enough times unlocks the amulet of the pit lord, dropping a table into a pit also unlocks something i believe. Thankfully, most of these things are tied to achievements as well, so if you want to make sure you unlocked everything you should check the achievements list & do them.
Make sure you do a run with beast mode turned on.
That is NOT his tail bruh