@dalto Thanks for that shellprocess hint, I got the installer working now 
@joekamprad Sry, I don’t have any repo. I just followed Unofficial zfs on root installation instructions (if you look at my last post summarizing all the needed steps) and just added this partprobe step to the installation process. So to summarize the addition again:
Add
- id: partprobe
module: shellprocess
config: shellprocess_partprobe.conf
to /etc/calamares/settings_online.conf and
- shellprocess@partprobe
after partition unit and before zfs unit in the exec: section and created /etc/calamares/modules/shellprocess_partprobe.conf:
# SPDX-FileCopyrightText: no
# SPDX-License-Identifier: CC0-1.0
#
# partprobe
---
dontChroot: true
script:
- "/usr/bin/partprobe || true"
i18n:
name: "Probes partitions for changes"
That’s all that’s needed (well the installation is not finished yet, still installing the system, I got only one CPU in the VirtualBox VM 
)