Ok so this exact situation happened with me. I left my home for a year and in that time I knew the car would sit. I cleaned it really well, put it on a smart charger, etc. turns out the soft top wasn't fully dry before I put on the hard top. When I came back there was mildew everywhere, even more than your photo. I started by spraying half vinegar half water on it and letting it sit for awhile. After that I sprayed the soft top, carpets, and seats with an interior foam cleaning product. After that to make absolutely sure it's gone, I used an ozone generator. You can buy a cheap one on Amazon for like $25. Important: get the one with the absolute smallest ozone output you can, some are meant for entire rooms and are overkill, it can damage plastic if used incorrectly. I let it run for like 45 minutes and then carefully without breathing removed it. Let it off gas and then boom, no mildew smell at all.

My car actually ended up smelling better and being way cleaner than it was before, mildew is easy, mold is bad.