but no one seems to discuss women who couldn't make such a move, how they might have survived on their husbands wages, or how they may have supplemented them.

Probably bc there were very few women who were unable to make such a move.

Those who lived in cities and were physically able to work in factories and such, did so. Those who could not join the factories usually lived in the countryside, either being farm labourers or working their own farms. Those who worked their own farms would have struggled without their men, but then this applied to many people. It simply meant working harder.

As for other ways of supplementing, there were the countless ways a woman during this time could keep afloat. Needle work, being a cook, renting a small cottage with a farm garden large enough for subsistance farming with potatoes, becomes a sales girl in a shop, ...

You also need to keep in mind that Germany has one of the oldest public welfare systems in place, parts of it date back to the 1880s.

If you cannot find any primary sources on the income of soldiers during WWI this is probably due to the fact that the two archives in Berlin that stored most of the records burned down in 1945.

Only the records of the Bavarian army survive in the Bayrische Hauptstaatsarchiv in Munich.