@Kadhrin I was in the exact same situation you are currently in. I had a spare Witch Hut base in all 3 of my cities from the original event (plus 2 artifacts in each city from the fellowship adventure which followed the original event). And then in the fellowship adventure after last years Halloween event I won 2 sorceries which I could redeem for either Witch Hut artifacts or Witch Sumoning Circle artifacts. I didn't need more Summoning Circle artifacts. And like yourself my old base was many chapters out of date and would require lots of RRs to upgrade. I chose to place the 2nd Witch Hut in all 3 cities at that point, knowing for now I could only evolve them to stage 5 but that more artifacts would enter the Spire in the future.
I think Witch Hut might be the best evolving building in the game which does not involve a pet food effect. Until Echos of the Forgotten came along, WH was the best producer of orcs in the game on a square for square basis. Echos does beat a WH for orc production, but the orcs are all it gives you. The Witch Hut is giving great orc production plus a decent number of Frogs and 3 KP daily. And if you care about culture, WH is very high on that also.
As far as getting the Royal Restorations I needed to upgrade my 2nd Huts I mainly used diamonds to craft them in the Magic Academy. To be honest, crafting Royal Restorations is my primary use for diamonds these days. My Magic Academy is at max, I've got 4 builders (don't see the need for 5), and I've already bought all the cheap premium expansions. My next premium expansion will cost 3000 diamonds. So roughly for the cost of a single expansion I was able to craft enough RRs to upgrade my 2nd Hut. Since WH gives roughly the orc production of an armory I felt this was a good deal. When you factor in the space required to provide the population it needs, adding another armory to my city would require FAR more than 1 expansion of space.