What you did in your youth will be with you the rest of your life. During childhood and adolescence when our body’s are growing fast, the muscles and nervous system is very plastic and adapt very quickly to any sort of stimulus. Sit on the couch watching TV and eating junk during those years and you will have missed a crucial developmental window that will never be completely reversible. In your case, you bike raced during at least part of those critical years when you were young enough and for long enough to cause a great deal of structural adaptations to occur in your body. Read the section in TftNA where we discuss Structural vs Functional adaptations and you will understand this better.
Those adaptations you made long ago are still there in your legs where as they never happened to the same extent in your arms. I see this all the time with adults who were runners, XC skier, or cyclists vs swimmers in their youth.
I was a swimmer during those crucial years. My arms/shoulders just don’t get tired even when I don’t train them. Whereas I have had weak legs despite training them hard for climbing, running and even ski race internationally during the past 40 years. Maybe its just me but I don’t thinks so as I talk to lats of folks with similar experiences.
As Eddy B said: Train your weaknesses and race your strengths. Your arms will probably never come up to the same levels of strength or endurance as your legs. You can try all those methods you propose (I have) but I doubt any of them will really make a huge difference.