Thank you @Joulinar - I’ve done as suggested and have been able to complete the update.
I’m having troubles with the ethernet not found after reboot, but have created a new post for this.
I wasn’t able to replicate the issue with curl. Did several attempts, including with Ondrej’s PHP repo added, with and without reboot and APT autoremoval 
. Sad as I was so curious about the reason for this.
However, I added the hint to the guide now, at least the solution is simple.
MichaelAlets
I could replicator the behavior using an ARMv6 Buster image on my RPi3B+. After the upgrade + restore curl was not working anymore. However a reinstall of curl fixed it in my case. 