Are you familiar with enabling the Developer Settings in Android Settings? Usually tap the Build Number 7 times in the ‘About’ screen within Android Settings, but it occasionally varies with the device model.
Once you’ve enabled the Developer Settings, I would be curious what options show under WebView Implementation for that.
I vaguely recall that you used to be able to switch between the System WebView and Chrome WebView / Firefox WebView, but I don’t recall which moons have to be aligned to do that.
Edit: I think it’s Android 6-9 that can use the Chrome Beta webview, then Android 10+ can use the WebView beta/canary from the Play Store. I suspect the device is running Android 5.1 based on the reported version 39 for the webview, so you might be out of luck there.
Alternatively, you might consider using Chrome directly on that particular device. You can use ‘Add to Homescreen’ to add an icon to your homescreen that opens SharpTools.io fullscreen. You could still use Fully Kiosk in the background for screensaver and motion detection features.