Yea so although the bands grant powers of a fullborn and those powers are significant but in terms of pure investiture, shardblades are way above bands. And medallions will not even compare a tiny bit.
What about fabrials and medallions then? Since fabrials have actual lesser spren trapped in them, they should have significantly higher levels of investiture as compared to medallions!
But an oathgate has only a bonded spren type of deal and not an actual trapped spren, so in and of itself, oathgates have much less investiture ? but they require shardblades to function, but they don’t actually consume the shardblades so the use of shardblade is more of a security feature only.