Your comment about no issues with no valid PTR surprised me.
Spamhaus PBL is build based on your ISP telling Spamhaus which IPs are dynamic and which IPs should not send email. Your ISPs seemed to be nice enough to allow you delisting from it, which not all ISPs will allow.
Abusix has a similar list, but its completely build based on dynamic looking or no PTRs. You can create an account and delist without any issues.
Never the less there is way more services than GMX that block based on dynamic or no PTR. A lot of smaller solutions have this option checked by default. And it actually has been a best practice for decades to have a proper PTR.
Spamhaus PBL is build based on your ISP telling Spamhaus which IPs are dynamic and which IPs should not send email. Your ISPs seemed to be nice enough to allow you delisting from it, which not all ISPs will allow.
Abusix has a similar list, but its completely build based on dynamic looking or no PTRs. You can create an account and delist without any issues.
Never the less there is way more services than GMX that block based on dynamic or no PTR. A lot of smaller solutions have this option checked by default. And it actually has been a best practice for decades to have a proper PTR.
If you can, I'd set one and be done with it.