For years, I've been using the Flickr Classic Plugin, and it's always worked beautifully, but unfortunately it's been discontinued...

So I need a replacement.
And I've been looking for one, and trying out extensions, since early 2020.

Finding an extension that can pull images from Flickr, and show them either as a gallery or as a slideshow is not as easy as I thought.

SmartSlider can do it, in fact I am already using it on a site that uses SmartSlider anyway, but I don't think SmartSlider does galleries. And anyway, it's way overkill if all you want to do is show a simple slideshow.

Then I tried SigPlus. Sigplus can pull the images from Flickr, no problem, but I don't like the way the layout looks at all. It can all be overridden with CSS, but that would signify an awful lot of work, and the result might still be a bit wonky.

Next up: Event Gallery. Overkill as well, because Event Gallery has a complete system in place for selling images - but it does in fact display a gallery very nicely. I also managed to create an autoplay slideshow. At first, the images jumped across the screen at an alarming rate, but I managed to tame it. Not as pretty as the Classic Plugin slideshow, but I probably need to spend more time tweaking it.
Event Gallery also works in J!4. But I am running into issues. Support is prompt and friendly, but so far I have a feeling the answers I get are written in a hurry, without really checking what the actual problem is.

I tried Jumazi. They have an extension called JMZ Flickr  API, and it pulls photos from Flickr. But it displays them in a very old-fashioned thumbnail grid and as far as I can tell it doesn't do slideshows.

I have tried RS!Gallery - and I had high hopes for it, because I use other extensions by RS!Joomla, and they are generally excellent. I'm sure RS!Gallery is a very good extension in it's own right, but unfortunately, it only has one way of accessing photos on Flickr, and that's by using their "Showcase Module".  Works well enough, but it's no use for me, because it selects photos based exclusively on tags. I need to be able to select albums. And I have too many use cases to consider tagging each and every photo that needs to be accessed.

Next: NoBoss Image Gallery. I did research this extension back in 2020, but I didn't persevere, because their own website was a bit messy, and the extension kept saying I needed to subscribe to access certain features, although this wasn't clear in their pricing plan (even though it's not expensive). Tried it again, in the spring of 2022, and found that their website had really improved. The extension still told me to upgrade from the free plan to be able to access Flickr - fair enough. Then it told me to upgrade to the next plan to access further features... This time I contacted support. And I got a very prompt en friendly reply the same day, including a link to a - fixed - new version of the extension. From that moment on, it's all working very nicely. It doesn't do a gallery as such, it's more of a slider, but it looks nice and works well...

I'm still tweaking settings - there are quite a lot of settings and it's not immediately clear to me what they do exactly.The extension is a module, so just adding a gallery to an article with short code is out. You need to create a module for every gallery you want to show. Also the module inherits some settings, such as the card layout for the module. If you want another layout, you'll have to add some custom CSS. 
But all in all, I'm happy with this extension. The gallery slider looks good, and the slideshows works very well (and it's very fast, too!).

There are some things I wish worked differently, the most important being that you can't dismiss an image in the lightbox by clicking outside the box. You need to click the closing X (and the X is not always easily visible on some backgrounds). And when you open a photo from the slider in the lightbox, you can't swipe to the next image from the slider, or set the lightbox to "carry on" as a slideshow.

And now for a mosaic experiment...