After having some problems using linux with tftp to install OpenWrt on Fritzbox 7530, I just want to share the info how it works.
Basically the problem was described here: Install OpenWrt on an AVM FRITZ!Box 7530 - #14 by boll (the thread is to old to add a comment)
The solution was to use Windows instead of Linux but that is not required.
Dont use Ubuntu 20.04 because it has no
python xinetd tftpd tftp packages available.
Instead use Debian 10 as a live system.
Then use this wonderful manual: https://spur-records.de/amv-fritzbox-7530-flash/
Works like a charm.