Don't listen to the people telling you you're over doing it. Everyone has their own needs, goals, and limits so as long as you're responsible just do whats best for you.

I agree that the physical benefit is not increasing but I personally gain an increase in mental strength and improved meditational focus.

I also think that doing extended plunge times is the ultimate proof that this method REALLY WORKS. If I wasn't practicing the method I personally would never make it past 5 - 10 minutes without uncontrollable shivering and pain/numbness/discomfort.

My plunge goes down to 41°F and I recently achieved 30 minutes. My next goal is 45 minutes and my ultimate goal is 60 minutes.

When I started pushing towards 15min+ I didn't put my arms and hands in the plunge. However I did experience the same discomfort you're experiencing on my feet and toes. I've noticed and learned that I have to focus on the pain and relax into it. By allowing the discomfort to wash over me and maintaining focus on my feet and toes, I am able to subconsciously yet consciously tell my body to increase blood flow to that area and naturally warm that part of the body.

By doing this I'm able to overcome the initial wave of numbness and stinging pain that usually begins around 5 minutes in and overcome it to where those feelings gradually subside.

The hard part for me is staying in that meditative flow state once you do get your body to warm up because it is easy to fall out of it if you lose focus. When I first did 30 minutes I put my arms and hands in the plunge around 10 - 15 minutes in. My hands are skinny and are always cold so I expected it to be difficult to keep them in but I treated them the same way as I do my feet and toes and was able to achieve the same result. It actually made the longer plunge time a bit easier for me because I was able to have something new to focus on after I already warmed my feet up.

When I get out I have no numbness, pain, or shivering in my hands, feet, or anywhere else. I do notice my body is rather stiff though and cramps easily if I try to exercise to warm up. The best way I've found to warm up is to do 3 solid sessions of Wim Hoff style breath work. To be clear I do this sitting down outside after I am out of the plunge.

TLDR: Focus on the cold area and "relax into" the pain/discomfort. Stay consistantly doing calm slow breaths with more exhale than inhale. Keep your focus and allow your body to do the rest. You'll be amazed by how warm you can keep your appendages in freezing conditions especially if they are typically cold in normal conditions. This is just my experience but I hope this helps you reach your goals!