Is the husband's failure to give his wife her legitimate rights, while still providing for her according to Sharia, after marrying another woman and after her consent to that, considered a violation of Sharia?

It is permissible for a wife to waive her right to equal division (of time, in the context of polygyny) for her husband or her co-wives, with the husband's consent, because the right is shared between them. This waiver is permissible for all time or for a part of it. The proof of its permissibility is that Sawda (may Allah be pleased with her) gifted her day to A'isha (may Allah be pleased with her). Similarly, the one who waives her right may retract her waiver in the future, but she cannot retract what has already passed.