I'm having similar problems with a DSLR and 100-400mm I recently acquired. Prior to this I had a Mirrorless Camera and burned through a series of long lenses. While I didn't have these problems, I was never happy with them. If I'd stuck with it, experimenting with settings, I might have gotten better shots eventually. I think with the entire photo creamed out, slightly out of focus, that yours isn't an autofocus issue. If it were, focus would look good closer up or further away. Manual focus would bear that out.
Your advice from these good folks seems like the ticket to rule out every possible thing, before thinking you have a defective copy. I think with factory QC, that's unlikely... but who knows? It looks to me like light challenges, and/or micro-vibrations, even that filter, clear or not. My micro 4/3rds was a fussy system.
I switched from native lenses with my mirrorless, to a high quality Apochromatic Refractor telescope on a tripod with remote shutter release. I was primarily shooting at home, perching birds (I live on a river with a nice Avian "green belt" up and down it's banks), so lugging the beast around wasn't that big of a deal at that time. Obviously, having no control over aperture was an issue, especially for video. I shot aperture priority and only modified ISO for light challenges.
This is the first thing I noticed. Magnifying the image using the LCD, and the button shutter, OR EVEN THE BREEZE OF MY BREATH, would cause a tiny, high speed wiggle, vibration, that always took a moment to go away. Remote shutter release and tripod was magic for my photos. I really think the impact of micro-vibrations is too often under-stated by folks.
For me with that camera I noticed three other things. From the tripod with that shutter release, Image Stabilization creamed out my shots. There was detail loss. I know your Image Stabilization is in the body, whereas mine was in the lens. I'm not familiar with your model so take this with a grain of salt. The second thing I noticed was that any filter at all was impossible. It impacted focus and I had dozens of shots just like yours. Any shade or cloud cover and it looked like Dusk. The third thing I noticed (this should be first because it happened with native lenses), was that autofocus was very difficult with birds in flight... impossible against the sky (for me), and too slow to acquire the subject, even with background/foreground to assist the camera.
I just got my first DSLR, as stated but now I read that they've finally come out with a 100/400mm lens for micro 4/3rds. With arthritis in my hands, needing a shoulder replacement as well, my poor skills holding that camera steady (it weighs a ton), I'm seriously second guessing my decision to go to DSLR, and wishing I hadn't sold My Gimbal Head and tripod. That's a honey of a new lens they've come out with.
This makes me think of one more thing. Have you tried another lens? If you have one that's good, it would certainly rule out all that fussing with settings. Try your factory, default settings, using the camera's own, point and shoot mode. Then, when getting a good shot, review the details tab on your File Properties. I considered that alone, a mini-tutorial. That's an essay you probably didn't need to read. Good luck to you. I'm wanting some for myself as well.