Musty smell will not go away in my used Prius

Hello, everyone. Two months ago I bought a used 2014 Prius from an older couple in New Mexico. But ever since I bought it, there has been this persistent odor in the car. Like a slightly musty carpet with a deteriorating adhesive/vinyl plastic/rubber smell. It is quite hard to describe. I was thinking it could be mildew in the seats, floor liner, or headliner, but I’m not sure. Doesn’t really smell like it. The car sat around a bit, but it was still driven ~5k miles annually in the last few years. It’s not completely overpowering or anything, but I would still like to have a neutral-smelling interior because otherwise the car is in excellent condition and has been very well taken care of.

So far I have tried:

• Replacing the cabin air filter. Big improvement as it was very musty and due for a change.
• Using the Toyota AC system refresh foam & spray. Just masked the smell.
• Spray with carpet cleaner solution and vacuum it up. Some improvement, but the smell eventually came back within a week.
• Spray with Tuff Stuff, scrub, then vacuum. Same result as above.
• Run ozone machine for 2 hrs. Zero effect.
• Remove the rubber floor mats and carpet from the car for a week and clean them. No effect, was still as strong as with them in the car.
• Remove the dash cover that was installed and scrape the adhesive off the dash. Mild effect? Hard to say.
• Leave the windows cracked for the past few weeks.
• Run my air purifier overnight twice with a carbon odor absorber. Very little effect. Still obvious when you get into the car the next morning.

I’m running out of ideas. My plan is to just keep using carpet cleaning solution and wet vacuuming it every 2 weeks until the smell is minimized. I’m very certain the smell isn’t from the AC system. Despite all the things I listed above, the smell is 70% as strong as when I got it. I’m looking for any ideas and would appreciate any advice from you fine folks. Cheers.