After testing my mod with the Poland DLC enabled, I noticed that the architecture sets of all civilizations other than Egypt got screwed up. I observed that the problem was not in my original mod but in the files for the Poland DLC (specifically its Culture.artdef file). Therefore, I've included an unofficial patch for the offending DLC file that should make it compatible with my own mod.
What you must do is:
1) Move the Poland_DLC_Cultures.artdef file from this mod's Artdefs folders into the ...Steam/SteamApps/common/Sid Meier's Civilization VI/DLC/Poland_Jadwiga/Artdefs folder (in other words, the Artdefs folder that came with the Poland DLC)
2) Rename Poland_DLC_Cultures.artdef to simply Cultures.artdef, thereby replacing the Cultures.artdef file already in the game.
3) This should "patch up" the Poland DLC so that my mod can work alongside it.