Epilogue: The new Kioxia KBG4 1TB has been working well for 24 hours - speed is better than the Samsung PM991 1TB and best of all it’s stable.
Update The PM991 is lying in a half-dozen pieces at the bottom of my trashcan - not safe enough to rely on for even backup storage.
Kioxia
Samsung
BUT the Sammy still overheats during Write operations. Problem is I don’t know when this actually started (before or after the big performance drop) and makes me think the Sammy should just be smashed and disposed.