I only tried "Wechselduschen" (apparently, there is no English noun for that. It's alternating between cold and hot water in the same session.) and I've seen tremendous benefits, similar to what you mentioned:

It took some time (like 2 weeks?) until I've noticed the benefits but I still vividly remember how much less I felt cold. It felt like my body temperature increased by a lot but in a good way. The cold showers also got less and less uncomfortable. They actually started to feel comfortable if not even empowering at some point. It felt like my body was a lot more effective and efficient when it came to regulating temperature by tightening and loosening arteries in extremities. This also makes a lot of sense since this is exactly what happens during the cold shower: your body tries to save heat by concentrating blood in your vital core.

However, unfortunately, like many habits, I stopped for some reason.

Need to start again.

Great post, should have received more attention imo.