This is a very common constraint, and unfortunately it's also what makes good center speakers so hard to build (or so I'm told). To that end, I would never buy a center for which there aren't independent measurements. I haven't seen any for the Arendal center, have you found any? I know Arendal posts a few, and so far the independent verifications on other units have been within a reasonable margin, but centers are just so hit or miss...
Because of the horizontal constraint, you're looking at a studio monitor, coaxial or $$$ like C426be. $5k is a hearty budget, though, and since you have Dirac you could even do something like 3x LS50 Meta crossed at 120Hz + another sub. I do think Kef is, overall, your best option. Simple, proven, within budget, easy on the eyes.
As an owner of most of the R-series, if I had to do it again I'd just get all used/b-stock R3's and better subs + full Dirac suite. But, a lot of that is because I know it's only a matter of time before I sell to try out something else and it's a lot harder to sell/ship towers. I guess I have commitment issues