Well there would be very few people who would state that at 18 or 19 years old a person is ready to take on the responsibilities of a 21rst century European constitutional monarch. So if she was to declare that in order to prepare properly for her role she would need to continue to study and learn rather than reign in person i think the overwhelming majority of the Estates-General would agree and support a regency for a few years. Once HRH turns 21 or is older that argument will be less powerful and support will dwindle with every year she is older.
Queen Wilhelmina succeeded to the throne at 1o years old and had to take on the reign herself at 18. Due to that the young girl was bombarded with not just the end of what we would now call primary education but in those short 8 years she also had to finish her secondary education as well as an extensive knowledge of Dutch constitutional law and Colonial affairs.
The same applied for her daughter Juliana who did have a small group of selected girls join her in the classroom for the primary education but later on Juliana had to be taught at home on her own to cramp in everything required before she turned 18.
She herself hated it and choose differently for her daughter Beatrix. Beatrix had lived through the Hofmann-affair at the Dutch court that could have seen her succeed at a very young age so wanted more time for her son and heir. That could not be arranged by a constitutional change as there was no political majority required for it but no doubt it was at that time that the solution of a regency was first mentioned as an escape for a young adult who succeeds to the throne before (s)he is ready and prepared.
That gave the younger generations the freedom to explore and train before taking on the role.
Aren't we taking that too lightly ?
If Amalia would declare that she is incapable to reign, she would have to justify such claim.
Before Parliament if not before a medical board.
The royal job doesn't bring just benefits, it also carries burdeons.
I agree that these days modern princes have university scholarship but, if her father was to disapeer, God forbid, too bad, she is 18, she would have to cope with her destiny and not say "Sorry I am incapable".
When would she decide such incapability would finish ? At 20, 21, 25?
Kings/Queens have duties before their people. Amalia might try to "negotiate" a lighter version of her duties at an early time, but if she doesn't want the "full package", she simply should step aside and let the next in line pass.
OK, different days - Victoria was 18 when she succeeded to the throne. D.Manuel II (not even the Crown-Prince) was also 18 when his father and elder brother were murdered in front of him, and he assumed a job he was not prepared/supposed to occuppy.
Nobody said being King/Queen is easy, although these days is much easier as monarchs have a lighter role than in Victoria or Manuel's days.
Majority brings responsibility.
You are no longer Daddy's little girl, you are legally a full adult resonsable for your acts.
Remains to know if you are up to deal with your destiny or you prefer to be an irresponsible teen-ager enjoying just the priviledges inherent to your birth.
The recent birthday party thrown by Mom and Dad with 100 guests when the NL were in lockdown comes to mind