As DynDNS asked me to pay (too much) for their services, I decided to look after an alternative and asked around to learn what is en vouge to use „nowadays“.
The options suggested to me were
- Using AVM Fritzbox connected service „myfritz“ and maybe a cname alias with any other domain
- nsupdate.info – looks nice, but does not accept any new users anymore (github)
- duckdns.org – the duck looks very friendly
- mooo.com – based on freeDNS
- dnsomatic.com – meta service that con update many other services at once
- dns.he.net
- desec.io – „Modern DNS Hosting for Everyone“ – provides much more than just dynDNS
- dynv6.com
- DynDns feature of LiveConfig
I ended up using dynv6 for now, mostly as the setup to use my personal domain was absolutely straight forward and their clean and well documented webUI was very inviting to me! 
One thing to note: The help page works, but is in regard of the fritzbox update URL not completly uptodate and does not show a HTTPS url. So it´s the following I would suggest to use
https://dynv6.com/api/update?hostname=<domain>&token=<username>&ipv4=<ipaddr>&ipv6=<ip6addr>&ipv6prefix=<ip6lanprefix>They also have a dynv6 community forum which I didnt need to use yet 