Can you concoct Kombu with Dulse (Palmaria palmata)?
The best konbu comes from mature plants but early in the growing season, is sun-dried over two days (or slowly and gently), retains a little moisture (still below mold-range if you have an aW meter) and then cured in a cellar for 6-24 months to develop flavour and loose it's briny taste and aroma. I learned about this in The Japanese Culinary Academy book 2, Flavor And Seasonings which I highly recommend.
You can definitely use other kelps. Here in Australia we have the very tasty golden kelp and bull kelp. I have tried this, even a few weeks of ageing helps. That white powder on the outside is manitol, a faintly sweet sugar. As the video suggests, you should keep it.
If the dashi from those packets you have is weak, I would think a longer steep in warmer water might help.
Good luck!