Okay, so as @sampsonf and @vgaetera suggested over at AskFedora](No network (IPv4?) connectivity via Ethernet at first boot for LUKS/Clevis setup - #2 by sampsonf):
…I did try it with the next stream now:
$ sudo coreos-installer install /dev/sdd -i ignition/fcc-minipure.ign --image-file downloads/fedora-coreos-34.20210427.1.0-metal.x86_64.raw.xz --delete-karg 'console=ttyS0,115200n8'
Copying image from downloads/fedora-coreos-34.20210427.1.0-metal.x86_64.raw.xz
Reading signature from downloads/fedora-coreos-34.20210427.1.0-metal.x86_64.raw.xz.sig
gpg: Signatur vom Di 27 Apr 2021 22:09:27 CEST
gpg: mittels RSA-Schlüssel 8C5BA6990BDB26E19F2A1A801161AE6945719A39
gpg: Korrekte Signatur von "Fedora (34) <fedora-34-primary@fedoraproject.org>" [ultimativ]
> Read disk 566.5 MiB/566.5 MiB (100%)
Writing Ignition config
Modifying kernel arguments
Install complete.
Booting…
And… voila… it does indeed boot without any problem!
However, is this really a solution? I mean next is unstable and… no one pointed me to an intentional fix for that, that would tackle it. (Maybe it only works, because of strange coindcidence?)
The only thing, which has been confirmed on GitHub is that Ignition will not wait for a network before trying the Clevis command/contact the tang server in the stable and testing stream.
However, that should not fully solve my problem, because well… there is and was no network even after waiting. 
(Or was it some kind of race condition?)
I’d still like to get an explanation for that…
But thanks anyone for helping so far, already, in any case.