KitKat is just another name for Android 4.4. I believe that Google also referred to this Android K while it was still under development before they came up with a nickname.

I have a Samsung Galaxy Tab Pro 8.4 that came with Android 4.4 that also had problems connecting with Bluetooth LE devices, in particular the Wahoo RFLKT. I resolved that problem by replacing the stock Android 4.4 with an open-source version of Android 6.0 called LineageOS (v.13.0). That fixed my Bluetooth problems.

Check out https://www.lineageos.org/ and https://wiki.lineageos.org/devices/

I see that there's a version 14.1, corresponding to Android 7.0, that may be available for your phone.

A word of warning though: you will need to unlock the boot loader on your phone which will void the warranty. Installing the wrong firmware may brick your phone. "Brick" as in making your phone as useful as one.

Buy a new phone if you find this too intimidating.