Below the Wireshark snapshot of the handshake compliant to the TLS 1.2.
The position 1 displays the enum ServerECDHParams where I can use any curve having the own ID RFC8422. I'm free for example to use Edwards, Brainpool or NIST curves having an ID assigned by IANA.
Please correct me when I'm wrong but at the position 2 I have to provide the value from the enum SignatureScheme which defines not only the signature algorithm (ECDSA in this case) but also the hash and the curve. What value should I then use if the ECDSA certificate contains a signed public key based on for example brainpoolP256?
The migration to TLS1.3 is not an option because the task was to extend the proprietary legacy server side TLS1.2 stack implementation to use the ECC.
Not using of brainpool is also not an option as this is a customer requirement.
Full specification of the domain parameters in the Server Key Exchage is hardly supported by any embedded TLS 1.2 client.