Just a thought of mine. What if instead of giving out echoes of umbra as loot, make a background story of each frame, and make them into a season of nightwave. After each season concludes, we learn the story of each frame and get the final rewards: an echo of umbra which you can install onto the specific frame whose story is told in the nightwave season.
The special feeling the players have toward Excalibur Umbra comes from the wonderfully told background story from the Sacrifice. Giving a piece of umbra as a random loot to make other frames sentient would not have that same impact.