Uhm, I’m not sure. I’ll have to think about it a bit and get back to you later.
I thought of the Celestial Workbench while building. I found it very annoying to set up crafting stations near where I was building or to be constantly going back to my base to craft items. I personally don’t think it’s overpowered since it’s an endgame item and doesn’t include all crafting stations. However, if it needs to be nerfed it could give access to less crafting stations (maybe only a work bench) or it could be placed instead of a permanent upgrade.
I think only a workbench would make it not very useful. There are a lot of crafting stations used basically only for decorations so I don't really see how it could be 'overpowered' per se. Plus as you say it's basically post-game content. It just saves time and space at a point in the game where the tedium really isn't a fun part of the ride anymore.
Maybe if it let the Sawmill craft every furniture item? So the Sawmill also gains the function of the Ice Machine, Honey Dispenser, Living Loom, etc...
That could be interesting and helpful. It wouldn't solve my problem though because I don't use specialty crafting stations very often.