This will work on stock Android, and equally well on CalyxOS, GrapheneOS or LineageOS.
This is only possible on Android version 12 or greater. For LineageOS, that corresponds to v19+.
Tap Settings > Security > More security settings > Encryption & credentials > Install a certificate > CA Certificate > Install Anyway and select your “Embassy Local Root CA” certificate.
Tap Settings > Security > Advanced > Encryption and Credentials > Install from Storage and select your “Embassy Local Root CA” certificate.