Would Egypt not have been extraordinarily difficult to defeat for the Romans? Even more so than Carthage?
I guess a lot of the downfall could be attributed to the decline of the Pharaonic family, itself. The first couple Ptolemies seemed to be quite good leaders.
Surely, the Egyptians could have fielded a formidable host to prevent invasion. Was it threat of Roman naval power controlling trade routes that the Egyptians feared? Just curious on the deeper intricacies of the whole scenario.
Would the Ptolemies been able to stand up the tests of time against the Romans had they had stronger, more unified leadership, as they had under Ptolemy I-III? Instead of infant kings and various regencies?
The main thing to realise about the Ptolemaic Kingdom is that they went into decline thanks to internal conflicts almost contemporaneously with the Second Punic War and the conquest of mainland Greece.
Their rivals the Seleucids actually invaded Egypt and occupied the entire kingdom with the exception of Alexandria where the Ptolemids held out while Antiochus IV was crowned Pharaoh in Memphis. It was actually only through Roman intervention that the Seleucids were turned back and the Ptolemids were restored to power...only to launch into another civil war with competing family members. Prior to that, a civil war erupted when Upper (Southern) Egypt appointed a new native Egyptian Pharaoh with the backing of the regional temples and 2/3 of the country effectively seceded for the better part of two years. This was due not just to a weakening dynasty but to the Ptolemaic practice of neglecting or exploiting the Egyptian populace to vie for control of farther territories in the Asia Minor and to a lesser extent the Aegean as well as hostilities with the Meroitic Nubian Kingdom who supported the Egyptian uprising in exchange for the Nubian cataracts which had been previously seized by the Ptolemaic Kingdom, all of which can be traced back to the first three Ptolemies.
Ironically enough, it was not so much fear of the Romans that brought the Ptolemaic Kingdom under Roman influence, but the political manoeuvrings of the various prospective monarchs. From Ptolemy VI right down to Cleopatra VII, the Ptolemids came to rely on Roman military interventions to settle their dynastic disputes, and the need to wield immense bribes to earn the support of otherwise reluctant Roman politicians is how they ended up in debt. Roman naval dominance was not really a concern for the Ptolemies from a commercial point of view because Alexandria remained the was ideally geographically situated as a hub of traffic from Asua, Africa and Europe. It would not see any decrease in volume until the construction of Portus and Ostia in the Imperial period but even these served to better handle trade centred around Italy than replace Egyptian port cities, and the two port systems worked in unison.
Egypt would almost certainly not have been more difficult than Carthage, for one thing Rome's reach and allies had increased dramatically, and even disregarding that, the Carthaginians were able to field numbers far in excess to those fielded by the Ptolemies.
I actually answered a very similar question "What were some reasons the Ptolemaic dynasty ended?".