I have two MacOS and two iPhones on which I would like to be able to use the same database with the Pro version. Is there a single subscription I can do, or which is the cheapest solution?
Versions for Mac and iOS are sold separately. You need one subscription for Mac OS and one - for iOS. I assume you have the same Apple account on all devices, because your subscription is linked to your account.
Yes, I have a shared Apple account with my wife in "Family". So just make a subscription for the Mac and one for iOS for the 4 devices, is that correct?
For each of these, can I choose whether to subscribe monthly or annually?
Yes. Only yearly subscriptions available at the moment.