A lightweight version of plasmashell is being developed by KDE:
https://github.com/KDE/liquidshell/blob/master/README

It is far enough now that it can be tested. It runs fine, let's you use your desktop without issues.
After installing, run it manually by stopping plasmashell and starting it:

kquitapp5 plasmashell && liquidshell

When you want to get back to regular plasma (on reboot you will have it, nothing changes on your install)

kquitapp5 liquidshell && plasmashell

Screenshot:

5 months later

I so wanted to use it or at least try it for a couple of days but the fact that the taskbar was so big really annoyed me, cuz my notebook can only use a 1366x768 resolution. I don't know why but I just tried it again and found out that switching the workspace indicators to display in a row instead of columns instantly slimmed the panel! 😃

To autorun it at login I:
- Added "Hidden=true" (without quotes) at the end of the file "/etc/xdg/autostart/org.kde.plasmashell.desktop". This prevents plasmashell from running.
- Copied "/usr/share/applications/org.kde.liquidshell.desktop" to "~/.config/autostart". This enables liquidshell to auto-start.

Slim panel screenshot: https://imgur.com/a/LMS3nva

Trying to build from latest commit found out that it now requires 'packagekit-qt5' for a system updates notification applet. Since we have octopi-notifier and I hate packagekit (and it's not in the repos) I removed the (un)necessary related lines of code and bum! it compiled.

This is the little patch I used (liquidshell.patch): https://pastebin.com/gDAS2JQC
And my PKGBUILD file ended up like this: https://pastebin.com/XDKcetzu

5 months later

I have now modified liquidshell so that PackageKit is only an optional dependency, detected in the cmake file