I get raw thumbnails with just libopenraw installed, but I use Thunar instead of Dolphin and my camera is Olympus (so they are .ORF files). I’m not sure which of those two details is the one making a difference in my case.
The ArchWiki page for Dolphin (in the file previews section) recommends installing raw-thumbnailer from the AUR, although it does appear the package has not been updated for a long time. That doesn’t necessarily mean it won’t work, but there is a comment which suggests you may need to manually update the PKGBUILD in order to get it to work.
I did find a Rust rewrite of this project though, which is likely your best bet.
You can install it from the AUR with Paru.
paru -S miniaturo-git
Press Q to exit the PKGBUILD review and proceed with the build. I just installed it to test and it only took 1 minute 44 seconds to install, so not bad.