If you're interested in learning how to develop secure software, I recommend checking out the free set of 3 courses from the Open Source Security Foundation (OpenSSF) on "Secure Software Development Fundamentals": https://openssf.org/training/courses/
They're hosted on edX. Like many edX courses, if you want to prove you learned the material you can pay to try to pass various tests and get a certificate, but you do not need to pay if you just want to learn the material.
Full disclosure: I'm the author. But hopefully you'll like it anyway :-).
Interesting...not heard of the openssf (I see they were only formed in 2020?).
Regarding secure software development, I've not seen many certs / exams covering the topic, so it's good you've developed those 3 courses. The only other cert I've seen is the CSSLP by (ISC)2:
This is one of those things. I do security engineering but it has little to do with building systems or software. In this case what they mean is "engineering securely" but a security engineer in a security team will be engineering various security tools and content (endpoint security tools/content, threat intel platforms,SIEM,etc...) so literally engineering security controls. Not that I mind but the ambiguity might cause confusion.
They're hosted on edX. Like many edX courses, if you want to prove you learned the material you can pay to try to pass various tests and get a certificate, but you do not need to pay if you just want to learn the material.
Full disclosure: I'm the author. But hopefully you'll like it anyway :-).