Version 20180830-1023 from the ISO, fully updated as of today. These are a couple of minor issues.

When I select "Documents," "Images," etc., under "Search" in the Dolphin places panel, it initially brings up an unfiltered list of the right kind of content (all images if I selected "images," etc.) before I type anything, but as soon as I enter a search term, it returns matches to that search term as if I had selected "all files" rather than the type I selected. It does this in all four categories represented in the places panel. If I expand the search UI in Dolphin using "More options," the "all files" radio button is selected rather than the one that matches the link I used in the places panel.

I seem to remember this working nicely before (I tried it out as soon as I saw there was something new over there compared to Kubuntu 18.04), so it may have been a regression introduced with one of the many updates since that ISO was created.

Also, when I try to set the default email client to anything other than Kmail in System Settings / Applications, it allows me to go through the application menu to look for a choice, but when I select "Thunderbird Mail" from the list, it leaves the field blank rather than populating it with Thunderbird.

Other than that and the usual nVidia woes (icon captions scrambled after resume from standby) that are already well known, it's quite solid. I have gotten some occasional crashes in Baloo, but it always told me the bug report would not be of any use to KDE, so it didn't send it.

Hi, did you try to look in the Dolphin search the option install other search tools at the right end? Maybe, you could add another engine.

Did you search Thunderbird from the System Settings / Applications (predetermined). I have this line: thunderbird %u

Hope it help you.
Regards,