Or they could go hypermonarchist and have the child be genetically engineered from both parents.

That would have made the Middle Ages really weird.

The Romans made it work, though. From ChatGPT:

The Roman Empire is the clearest example of adoption used deliberately to choose capable successors:

Nerva → Trajan
Nerva adopted Trajan, a respected general, who became emperor in 98 CE.
Trajan → Hadrian
Hadrian was adopted (likely on Trajan’s deathbed) and succeeded him.
Hadrian → Antoninus Pius → Marcus Aurelius
A chain of adoptions: Hadrian adopted Antoninus, who in turn adopted Marcus Aurelius.

👉 This era is often called the “Five Good Emperors,” largely because merit-based adoption worked better than hereditary succession.

I see someone hasnt watched Young Royals

This is the Habsburgs, they can afford a surrogate