Ionisation enthalpies of $Ce$,$Pr$,and $Nd$ are higher than $Th$,$Pa$,and $U$. Why?

(N/A) The ionisation enthalpies of lanthanoids ($Ce$,$Pr$,$Nd$) are higher than those of actinoids ($Th$,$Pa$,$U$) because of the following reasons:
$1$. The $4f$ electrons in lanthanoids are more effectively shielded by the inner shells compared to the $5f$ electrons in actinoids.
$2$. The $5f$ orbitals in actinoids are more extended in space and have poorer shielding effect than $4f$ orbitals.
$3$. Consequently,the effective nuclear charge experienced by the valence electrons is higher in lanthanoids,making them more tightly bound to the nucleus compared to actinoids.