Is it permissible to donate 70% of one's expenditure secretly for the sake of Allah, and 30% publicly for the sake of Allah as a form of promotion and advertisement?

It is better for voluntary charity to be concealed, as it is further from ostentation (riya'). As for obligatory duties, it is better to declare them. This is supported by what was narrated from the Prophet, peace and blessings be upon him: "The best prayer of a person is in his home, except for the obligatory prayers." And Allah Almighty says: "If you disclose your charitable expenditures, it is well; but if you conceal them and give them to the poor, it is better for you." However, if there is a legitimate benefit in displaying charity, such as encouraging others to spend, while the giver is secure from ostentation, then displaying it might be better. Otherwise, it is not permissible to declare it if the intention is fame and reputation.