Which would you use to replace Picasa (on a Mac) ?

Darktable's targets digital photography enthusiasts or professionals. The learning curve is steep. Picasa's target demographic was "everyone". So probably Digikam.

I can confirm. Every now and then I want to edit some of my photos and run Darktable, only then to remember it has zero automation and a hundred of sliders for individual adjustments. It definitely requires dedication to use, more of that than I have to give for holiday photo editing.

You can get by using the same 5 sliders for almost all your photos. The other sliders remain available for edge cases or when you want to go the extra mile.

> lots of import/export stuff, tagging

So perhaps closer to Photo Mechanic?

Would you say I replaces iPhoto/Photos on Mac?

I'm not sure how seamless the experience is on macOS, but I'm happily using digiKam on Linux as an iPhoto/Photos replacement.

It's nowhere near as polished as Photos, but it's capable, open source, and multi-platform. I hope to maintain my photo library for decades, so avoiding proprietary software / service lock-in was my most important consideration. This decision was reinforced earlier this year when we had to do some surgery on my spouse's meticulously curated Photos library: it's damned near impossible to completely preserve metadata when moving items between Photos libraries, and it is impossible to remove RAW files from stacked RAW+JPEG pairs in the Photos library. If I ever ran into those issues in digiKam (which I haven't), I'm confident that my SQL abilities would be sufficient to work around them.

FYI Photos/iPhoto also store their metadata as SQLite

Thanks, I will try Digikam too.