I couldn’t quite solve the problem yet, but I kind of forgot that HassOS is build upon alpine when asking the question.
I ssh’d into my Home Assistant setup, added my ca to /usr/local/share/ca-certificates/ and ran update-ca-certificates afterwards. I also verified that my certificate was correctly appended to /etc/ssl/certs/ca-certificates.crt .
Sadly to no avail it seems, as all HomeMatic devices remain unavailable when using the verify_ssl option.
Does Home Assistant maybe use a different certificate store?