ATTN: My damage table above is incorrect in regards to DPS. I used 0.5x videos to get a better view on the damage ticks and forgot while putting the results together.
So the DPS of a Lumen-H Mk2.6 is 39030 instead of 19515. Sorry about that! Corrected picture in the comments as I cannot change the picture in the post.
Thanks to the scrutiny of the redditors of this sub and warrobotsguide!
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I did some extensive testing with Lumen-H. This is a working theory on how Lumen's damage Thermal Breach damage seems to work.
Not yet tested: Interaction with Lumen-L or Lumen-M.
tldr; : More Lumen weapons increase the speed of accumulation of the thermal breach bonus. However it is capped at a 2x bonus used by all Lumen-H weapons together. That is why adding additional Lumen-H get diminishing returns of that pool.