The real breaking point for his brain (it was never in good condition) was Covid, and there is actually quite a lot of evidence out there to support this. He became even more reclusive and paranoid, distancing himself almost completely from the bureaucratic machinery and most of the elite.
"putin is fucking insane" is something a lot of american chin-stroking think-tank/academic and beltway types don't really want to reckon with for any number of reasons (not least of all it would require a massive reevaluation of the last 30 years of their work) unfortunately, it remains true