The OP says:
You can do this one in one of two ways (but they are mutually exclusive, because you have only "one bite.")
Given ONLY the two choices the OP provided; the better option is legislated equality. The "ascension" option, after the death or retirement of the enlightened Monarch, is far more likely to lead to the immediate assassination of the female successor, or her forced marriage to a Male that then becomes King. In a world of female subjugation, the vast majority of men (the only ones with a vote) blinks twice: They don't think the Monarch was enlightened, they think his wits were addled and he should have worked harder to find a good successor that would care for the females in his charge: They don't admire him, they think him a traitor that left a power vacuum into which violence was bound to rush, and the consequence was fatal and bloody to his own family, and instability followed by a coup in the government. The addled Monarch will be vilified, not praised, and unsuccessful.
Personally I am for 100% equality; but putting myself in the shoes of these fictional characters, their reactions are not pretty and a female monarch is unlikely. An alternative to violence would be overriding the Monarch and appointing a male Regent (like a child would) that would actually run the country, an "acting" Monarch, until some male was born in the Monarch's bloodline.
Then the infant King would be appointed a Regent to run his affairs. In the end, such arrangements can result in a complete loss of power to the Monarch; the Regents and some committee (like Parliament) use the opportunity to parcel out the power of the Monarch which is no longer protected with force by loyalists to the Monarch.
Remember the psyche of all societies that subjugate women; both boys and girls are trained from birth to believe women are not warriors, and not thinkers, and not brave, too weak and slight to be able to defend themselves, that women are addled and physically disabled by menstruation and pregnancy. The female Monarch may be physically protected by a guard of men willing to sacrifice their lives to protect her, but they are not going to protect her political power because they are warriors, and do not believe she is capable of making intelligent decisions, or commanding an army, or even dealing with foreign powers from a position of strength.
Her guard may succeed in preserving her life, but they won't obey her commands any more than they'd obey a five year old: If she has a loyal guard, they will bargain for her life (and income and their own jobs) by endorsing a Regent, Husband (in waiting if she is very young), or new King of their choosing. Because of her own culturally instilled beliefs, she will likely agree to such arrangements immediately upon realizing she faces drawn swords if she doesn't, and while the people may be happy to keep her in a jeweled carriage as the Monarch's Queen or Princess, on his strong arm at Parades and Parties, they won't back her as Monarch.
The Monarch's female ascension plan is stillborn.
That said; earlier comments and the accepted answer cover the specific forms of legislation I would also consider priorities; I will vote for some of those.