The obvious candidates:
Dinbren - short bouldery/power endurance routes. A delightful outlook. A handful of 6s, quality starts at around 6c+/7a.
Devil's Gorge - the best climbing in the Clwyd IMO but conditions can be fickle. Quality starts at 7b.
Llanymynech - half decent 6s and low 7s in the main quarry. The climbing won't blow you away but maybe worth it for the novelty of 35m+ routes. Now the bird ban is lifted, Red Wall is good in the 6c-7a+ range. Very 3D climbing and has quite an adventurous feel.
Great summary, thank you.
Further to this:
Dinbren has some steady trad in the E1/2 range.
Other good trad crags in the area (aside from Craig Arthur which is a bit bigger and more intimidating) include: Maeshafn, Pot Hole, World's End and Pinfold - all shortish and relatively minor-ish but loads of choice in the mid-grades especially well protected stuff. You can get a LOT done in a day mileage-wise.
Devil's Gorge is a really cool location but seems to make The Diamond look reliable for conditions (although it's not that far to walk in and out and divert to other crags). Maybe best at the end of a heatwave with an incoming storm front.
Llanymynech also has a bit of genuinely good trad E1/2. The Red Wall - imagine in you ate a mixture of red dust, grease, and Avon Main Wall, then regurgitated it in a particularly blocky and chaotic sort of way. You'll love it.
There's one nice E2 at Trevor and I'm sure some of the sport is fine at quiet times.
Edit: I would very happily meet you at anywhere in Clwyd Lime. Apart from Trevor, fuck that shit.
Edit 2: Not that my "meeting duncan for climbing" track record has been particularly good in the last year, but I have only had.....2 full-shutdown nausea days this year, I think.