You can use an expect wrapper around nix repl that does what you need after it starts. The following always imports builtins:
#!/usr/bin/env nix-shell
#!nix-shell -i expect -p expect
spawn -noecho nix repl {*}$argv
expect "nix-repl> " {
send ":a builtins\n"
interact
}
But you can see that it’s easy to add more commands 