Digikam was my replacement for photo management and tagging. And darktable for editing. You can launch darktable from the Digikam catalogue, so it’s pretty smooth.

It took me a while to get used to darktable, especially the masking, but I genuinely prefer it to lightroom now.

You can also use other editing apps, like Gwenview, etc for quick and dirty edits. The only thing I’ve had no luck with are things like Topaz and DxO. They won’t run on Linux and don’t really have equivalents.