as a fellow gamer on a budget i sympathise with you, but at the end of the day LDA is a business not a charity. all the Gold serials cant be given away free.
as for your wish for "more forewarning": i'm not sure where you read of the Beta offer after googling the game, but LDA have stipulated right from day one that the serial beta offer was for a limited time only, and then there was a weeks notice here on their site that the offer was closing.
as i said, i do sympathise, i really do. but a moments research by you before buying the original, would have told you that the beta offer is no longer valid.
there is going to be an as-yet-unspecified upgrade offer from PoM to Gold, and Hegemony: Philip of Macedon is imho a great game in itself and well worth the special offer price on Steam atm. the "constant bombardment" of the AI can be dealt with by your strategy rather than just constantly fighting fires: go and find the source of the attacks and position your boarders at easily defended cities by expanding till you find them. dont over-extend yourself and dont put "all your eggs in once basket", spread your forces around stategicaly.
i do hope you give PoM another go and stick with it, as otherwise youre depriving yourself of many, many hours of great gaming.