Ive been making soft pretzels for years. Ive switched to Alton Browns recipe from this website.
Homemade Soft Pretzels Recipe : Alton Brown : Recipes : Food Network
Soft pretzels cook kinda like bagels. They need to be boiled in the baking powder solution ( as you mentioned) but as I do ( and in the recipe) you do this for 30 seconds per pretzel( I flip them 1/2 way through the boiling process)
Also, the temp to cook them at in the recipe I used is 450 F for 12 - 14minutes
I not sure why yours are hardening up so much at 220. Ah, your at 220 C :) which is about the same temp. Make sure they are a decent size. The only thing I can guess, is maybe your pretzels are too thin or flat and are dehydrating during the cooking process.
As a tip, what I do after they are done cooking, is I melt some salted butter and mix in a little sugar to give it a very slight sweetness to it, and brush it over the pretzels.