WHY CAN"T YOU COMBINE SEASONAL SHARDS!?!?!!?!?!!?!?!?!?!!? This makes NO sense at all. They aren't even on the forging chart. This is BS
The page for combining shards is designed so that it is already full with the current shards in the game. If you want to combine seasonal shards, you need to go to the second page for combining shards (at the tech priest). On Xbox, think it was RT (right trigger) to get to the second page.
Archeotech are of the highest level, you cannot combine them and get an even higher tier. In case I misunderstood your question, please let me know.