Which Apple Watch is compatible with iPhone 12 mini?

My Ultra 2 is paired to my iPhone 12 mini, so all new Apple Watch models that have watchOS 11 preinstalled, are able to pair to an iPhone that has iOS 18 installed.

Also have a look at the related article:

Apple Watch and iPhone compatibility - Apple Support

though some people report issues getting the older 1/2/3 models to connect with newer phones

also you should not buy older than apple watch 6 (including SE 1g)

if you wish to have the watch support the latest version of watchOS which is 11

Apple Watch - Wikipedia

The iPhone 12 mini can run iOS 18, so provided that you are willing to update to that, my guess would be that the iPhone 12 mini is compatible with all Apple Watches to date.

But I’d suggest checking the Tecnicsl Specifications for the watches you are considering.

Manuals, Specs, and Downloads - Apple Support