Since the oily mayonnaise sold in stores always feels like it's clogging my arteries, I for something to use as a substitute.

As I suspected, tofu mayonnaise is delicious.

This will keep in the fridge for 2-3 days.
Experiment with it in different dishes.
If some liquid rises to the top, you can either discard it or mix it in again.
Instead of pickled plum powder, you can decrease the rice vinegar by 1 tablespoon and add 1 tablespoon of pickled plum vinegar.
Using [1 Tbsp rice vinegar +1 Tbsp pickled plum vinegar] is also yummy. Recipe by Mikikimiki