An update from the original poster, mainly for anyone later searching for info on this topic and finding this thread:

Following the advice from "cellery", I ended up getting a Cateye Strada CC-RD410DW. It's not nearly as good a cadence computer as the wired Cateye Astrale 8, but seems to be the best wireless standalone non-GPS non-cellphone computer available for someone interested primarily in monitoring cadence.

Pros: It works as described. Cadence can be set as primary and largest display, with a choice of speed (and heart rate -- see cons below), odometer, trip odometer 1 or 2, etc. as secondary (smaller) display. There are a bunch of useless icons in the corner of the display, but they are small and not (to me) distracting. The unit I bought (from REI) came with the dual cadence/spped sensor already paired with the computer, and it just worked out of the box once mounted on the bike. Mounting is pretty easy, and sensor positioning wasn't overly finicky. It comes with a standard strap-on crank magnet, but works fine with a rare-earth disk magnet stuck on the inside end of the pedal shaft (which is a stronger magnet and allows a larger and less precise offset between the sensor and the crank). In the first 500+ miles, i have experiences no apparent loss of signal or interference on a bike with a front dynamo hub and always-on headlight and taillight. Clever mount allows mounting at right-angled orinetations, so you can mount it on e.g. etiher on fore-and-aft stem or transverse handlebars. (Stem mount is a nice centered position not all computers allow.)

Cons: The most important negative (for me) relative to the wired Cateye Astrale 8 is that the Strada Wireless is slow and delayed in updating the cadence dsiplay. I suspect that this means it is displaying average cadence over the last 10 revolutions or 5 seconds or so, rather thanh the last 2 revolutions or 1 second with the Astrale 8. Thta might be deliberate, and you might like it if you wanta smoothed-out diaply of average cadence. But if you want real-time cadence to judge shift points or fluctuations in cadence (e..g to maintain steady cadence), this isn't nearly as good. Increase your cadence, and the display gradually steps up to the new cadence after several seconds lag.

Other minor cons: Even if you don't have a heart rate senson, the secondary speed display is split between speed and a "0" for heart rate. There is one fewer significant digit on the display than on the Astrale 8: Cadence and speed are shown to the nearest rpm and mph or km/h, not tenth. Odometer is in miles or km, not tenths. Trip odometers 1 and 2 are in tenths, not hundredths.

I haven't had it long enought yet to judge battery life.

I paid full price and have no commercial interest in Cateye or any competitor.