Hi Scott
With European royalty the general guideline has been to use their dynasty name as the Last Name at Birth (LNAB) eg. Plantagenet, Tudor, Capet, Carolingian, Bourbon etc etc.
The other option is to use the country they ruled, but this should be in their own language not necessarily English.
Almost all major royal monarchs should already have profiles on WikiTree so if you have trouble finding any just post on G2G as you have done. If you add the tag euroaristo, this will attract the attention of members of the European Aristocrats project.