any idea why this isn't a great hall.

[Hechicera]

That would work, but may meddle with moral bonuses needed when using them as straight decor later. The change would have to be made to any plant findable or randomly printable before the cycle number they want to be the new challenge level.

I mentioned in the bug thread that the plant changes opens up a balance can of worms with decor->morale->new dupe system. I was testing on an easy beginner asteroid intentionally. Great Hall in that case may be the most accessible first major upgrade needed to use dupe skill points. The food changes also play into this loop. Experienced players will know what their options are and some may enjoy new challenges. But that change could mess with an existing an already contentious balance lever and hit newbies early game, so I advised caution.

The other option would be a decoration(s), only craftable with a certain art skill that are specific great hall items. My preference that way would be based on the dupes art skill, not skill point level. More ways to reach a goal are always nice. (So pick a low art interest and train or grab a single trait dupe w/+7 to rush).

Either your suggestion or this would be preferable to a contrived and unclear rule. Though detailed in game tooltip changes would go a ways to making it more clear. Even now it isn't clear that it is the base decor item that must have the specific number. Not the modified total after adjusting for material. IMHO, if you need tooltips that detailed to explain it still in early game, it may not have been a good choice.