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:
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 :-).