For Harry to become King, the Queen, Prince Charles, Prince William, Prince George, Princess Charlotte, and Prince Louis would all have to die (although Harry would become Regent for George, Charlotte, or Louis until they were of age, he would not be King).
If Harry then died, while Meghan would be a Queen, she would not be the Queen. Archie would become King (possibly with Meghan as Regent, but the law would need to be changed for that to happen - otherwise Andrew would be Regent under current law).
There is no mechanism in British law for Meghan to ever rule as Queen - she could only be a Queen Consort to Harry with no power of any sort, or potentially a Regent to her underage child.
No matter the love/hate relationship the Brits have with the monarchy, there is a zero probability a mulatto grifter will be allowed anywhere near the throne as Regent or otherwise. Baby Succotash is a non-player, as well. Fortunately, Migraine Markle-Farkle wouldn’t want any job that looks like work.
So Prince William will be next? If THe Queen and Charles both expire?
The Odds are high but it is possible for that to happen