Housing decor items will have a 100% drop rate

https://www.warcrafttavern.com/wow/news/where-to-get-player-housing-decor-in-world-of-warcraft-midnight/

100% drop rate for decor from dungeon/raid bosses. You only need to go back if you want multiple.

Also worth noting that every profession is going to be able to craft decor from every expansion, so now's a good time to start leveling your professions through the expansions you skipped over.