I have an iPhone 6S that I just inherited from my wife to see if I wanted to switch from Android to Apple. It has been upgraded to iOS 14.3 and has a battery with 99% of its capacity left as Apple replaced its battery just before the wife upgraded to a newer iPhone.
My iPhone 6S works great with the ReSound Smart 3D app. My Android phone is a 3-years-newer 2018 Galaxy Note 8 running the Microsoft Launcher and even though the Launcher interface and most apps are very snappy on my Note 8, the behavior of the Smart 3D app on the iPhone 6S is great, very snappy compared to the Galaxy Note 8. The reason for switching to the iPhone 6S was to test how I like the Apple Watch 6 controlling my HA’s through the Smart 3D app running on the Apple Watch but linked to my iPhone. That works great, too. So I would say, based on my experience, that your problem might not be that the iPhone 6S in general does not work well with the Smart 3D app or iOS 14.3 but rather, perhaps, your particular individual iPhone 6S may be showing its age. My iPhone 6S has sat unused for several years before I asked my wife to let me have it to try out an Apple Watch and she gave the phone a lot of TLC when she used it. The other critical difference is I am running rechargeable Quattro 961’s. So perhaps the ReSound app does have a problem with disposable zinc-air batteries but I don’t have any problem with the iPhone 6S monitoring the charge state of my rechargeables. I mentioned in another thread that there is an iOS app called Battery Life for both the iPhone and the Apple Watch. It’s intended to monitor the charge state of the iPhone and Apple Watch batteries (one from the other) but somehow it also picks up the charge state of my Quattro Li-ion batteries from one or both HA’s in a somewhat flaky way. It might be interesting if someone tried the Battery Life app with ReSound HA’s using disposable batteries and reported what it does. The app is offered on the “freemium” model. You get some basics for free but if you want the whole enchilada, it costs something like $4. Battery Life on the App Store (apple.com)
Good Luck with the Smart 3D app. There might be a problem with disposable batteries but otherwise I can say the app, the iPhone 6S, and iOS 14.3 work great together and also great with an Apple Watch 6 running WatchOS 7.2 although as others have noted, you can only run the Smart 3D app thru a complication on a more modern Apple Watch face like Infograph by creating an Apple Shortcut to launch the 3D app: Apple Watch - #11 by jim_lewis
Edit_Update: Something else I do that might make a difference in the general behavior of one’s iPhone. In Settings under Battery, being the battery-conserving-monger that I am, I always run my iPhone in Low-Power Mode to keep ~all apps that would like to try from running in the background. YMMV, but no text, e-mail, Whats App msg, etc., is so important that I have to be notified right away (and I still seem to get notifications anyway from certain apps like Ring and Outlook). So if you have a lot of apps that have requested permission to do something in the background, that could be a difference in performance between your iPhone 6S and mine. I tell ~all apps that they’re only allowed to use my location when I’m running the app, etc.