socket encryp and decryption between dart and c++

I work on Chat server in c++, data tranfser between client(dart) and server(c++), i want to use AES encryption for data and i have many question

1_ Which one should I choose? AES-GCM or AES-CTR, If you prefer another method of doing this, please explain.

2_ I know of several library and class for this, such as RSACryptoServiceProvider Class , Crypto++ which one do you think I should use, or if you know another library or documation , name it

3_ In my previous experience, i changed data like message and message information to string, encrypted string , add the first and last encrypted string with a special character so that I can get the healthy data and finaly i decrypted in server or client and it worked well, my question here is this the standard method? because i have heard elsewhere that each byte must be encrypted separately and sent with a socket, not collectively.

4_I did not find a good library for aes inside the dart, and my question is that I can write the class for decrypt and encrypt data in c++ and use in dart ?

thanks