I would also really like some sort idea of just how easy or hard a material is to get odor out of.

For a while now now I’ve just had to have multiple backpacks and rotate through them when one becomes too fragrant.

If anyone has some guidance on how to get human ecology bacteria (i.e. that funky stank) out of synthetic materials that doesn’t result in smelling even worse of the cleaning agent, I’d love to hear it.

I haven't ever had an issue with stanky outdoors gear, but I've had luck removing stains from different gear by washing with either regular laundry detergent or "Oxygen-based cleaners" (oxiclean and knockoffs) as they have a different mechanism of cleaning, and drying in sunlight and fresh air.

Have you tried Defunkify?

https://defunkify.com/

I haven’t used it myself but a friend swears by it.

Vinegar