What brand are they?
I don't know if I'd blame the shoes for the brake fade. The brake fluid is getting too hot and boiling causing brake fade/spongy feel. That heat is generated by the shoes grabbing the drums. It seems like the shoes are doing their job. The drums are what is supposed to dissipate the heat. Are your drums new ? If they're original and cut to the limit or beyond, they aren't going to dissipate the heat generated by the friction of shoe on drum.
If you have to ride the brakes in the mountains you probably should be in second or even 1st.
