Hi @e8kor , yes, unfortunately the maintainers (myself and @pkinsky) have moved on from Nitro. Additionally, Nitro has moved away from supporting this project. I have a fork of it at github.com/malcolmgreaves/avro-codegen. I unfortunately don't have the bandwidth to add new features. But I'll maintain it as best as I can on my own time.
For your original question, I do not understand: the GeneratedMessageCompanion type class does the encoding and decoding. What do you mean "tried to implement avro serializers/deserializers base..."?
Also I have never used avro w/ C. I have no idea on how to do this either.