Sourdough revival - making it *more* sour?

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Hi all,

 

I've had stale starters sitting in my fridge for over the past year or two hahaha. I'm really horrible at regularly feeding them so they develop the black/greyish liquid that I end up draining every so often. Yesterday I pulled a couple out to revive (drained and discarded over half probably) and mixed in approx equal parts of AP flour + water. I got close to doubling what was in there leaving the jars out overnight. I just tried frying up some of the discard this morning and it wasn't bad but I feel like it could have been more sour....at least, I'm assuming I should be able to get my starter to that point (where it's tangy). Any tips on how to get [revived] starter back to a state where it's pretty sour and tangy? Should I feed with whole wheat flour or something else?