An easier solution is to boot Steam in offline mode and then open the mod. Same thing for all non-Steam Portal 2 mods, actually, since this issue is caused by the game thinking "I'm subscribed to workshop maps but none exist in this maps folder, therefore I must download them all."
But the best solution (at least on Win 10/11 and I think Linux) is to use a symlink. You can make a special shortcut in the mod's maps folder to make the game redirect to the normal workshop folder, and therefore not try to download anything. HowToGeek has a tutorial on making symbolic links. The soft link (the /D one) works for me. You have to run the command individually for every mod to generate its unique shortcut, but it's super worth it to not have to worry about redownloads or going offline.
WaterfallAni
Ani joined
I like Portal.