I do the Nautilus circuit weight stations at my gym. They are not free weights but are the machines...so my weights might be 50 pounds of resistance or higher.
I also was embarrassed to wear sleeveless tops. I found after a year that my arms looked much more defined..not perfect but muscular. I do around half an hour of weights.
Do you have access to a gym?
It sound like you have a good exercise program.
The only thing I can think of is to expand your arm exercises with a few different ones.
Maintenance 3 years!