Dashboard is history in Catalina

I think you can go to Keyboard in System Preferences then to the Shortcuts tab. Choose Mission Control from the list on the left, then on the right both Show Notification Center and Show Dashboard can be assigned keys. I say “I think” because I’m using a Mojave install that has had settings migrated from the El Capitan machine I’d been using for years, so it’s possible, although unlikely, that it can only be configured that way on older versions of macOS then migrated, but it’s worth a try.

I don’t think that works quite as seamless as it might sound.

You can assign F4 to Dashboard, but then you have to hold fn while hitting the Launchpad key to actually get Dashboard. Unless of course you’ve set up your Mac to always default to fn keys, but then you’d already be pressing fn to get the overlay function anyway. Apple does not IIRC expose the Launchpad key overlay to that sys pref so you cannot repurpose it (unless possibly with 3rd-party tools). I think we actually went over this very problem on TidBITS Talk a long time ago.

I use an Apple Magic Keyboard with Numeric Keypad on my iMac currently running Mojave. The F5 and F6 keys do not default system functions assgned to them. I assigned ’Show Desktop’ to F5 and ’Show Dashboard’ to F6. I can execute those functions by hitting the appropriate keys without hitting the fn key.

However, I believe that laptops without a Touch Bar assigned those keys to Keyboard lighting, so that functionality is not available there.

You’re absolutely right, Alan. I should have mentioned I had a MBP w/o TouchBar in mind.

On those you can choose to either default to the overlay (media keys etc.) or to fn keys. If you choose the latter you can assign whatever you want, but you then need to hold fn if you want to access an overlay. OTOH if you default to the overlay, you cannot reassign that without using the fn key (because effectively you’re reassigning the fn key, not the overlay).

Ok, I don’t have a laptop with a TouchBar and didn’t know it worked like that. On the four keyboards I use that all have real function keys, assigning Show Desktop to a function key without a builtin function works without simultaneously hitting fn, and assigning it to a function key with a builtin function works by hitting fn-[key].

However, you don’t have to use a function key. For example, I just tried assigning command-option-` to Show Dashboard and that works as expected and is pretty easy to type, so there are a lot of choices if function keys don’t work or aren’t convenient.

And, just to complete this discussion:

On a keyboard with TouchBar, one of the options for the short and extended control strips in Mojave is ‘Show Dashboard’.

I am holding off on Catalina indefinitely. To get a single key to control Dashboard overlay, on iMac, I have checked System Prefs > Keyboard > Use … standard function keys. Then in Mission Control prefs, I have set Dashboard: As Overlay, and Show Dashboard: F12. I have also set Keyboard > Shortcuts > Mission Control to be sure it agrees.

If you’re a mousy person, in Mission Control prefs you can also set up Hot Corners for Dashboard or Notifications or several other doo-dahs. (Personally, every time I set up a Hot Corner I find myself constantly triggering it by mistake.)