For me personally, it's the updated breeze theme. But I might not be completely objective here, since I drove this effort quite a lot :)
Some screenshots to see the difference: https://invent.kde.org/websites/product-screenshots/-/merge_requests/51/diffs
It is part of KDE Gear, so generally in the gear release announcement. The last few releases were not that big in term of feature but the next one includes some goodies.
KDE Connect will have the Bluetooth backend enabled by default with the next gear release (24.02). I'm not sure if this is BTLE or normal Bluetooth.
No idea :(
For me it is pretty instant. I never miss my Bereal notifications thanks to it :)
Yes
No idea :(