I've played Valheim for some time now. I realy enjoy this game but at this point I dont know how to update the Workbench to level 6.
The workbench itself says, it needs to be updated to level 6 to get new item upgrades. At this point I dont know how to update it.
There are multiple updates for the workbench in Valheim. You listed those correctly in your question. This is the right way to fully updated your workbench:
Currently there is no workbench level 6 available in Valheim. It will be added later into the game. The workbench shows the available updates for level 6 but you cannot reach them for now.