Regarding Seljuks, the key to their eventual success was never sheer military might. The Byzantines DID have victories over the Seljuks and Turkoman raiders. The problem was that they couldn't consistently keep their army in the field while the other opponents in the Balkans were at play. Byzantium had to fight multi-fronts against multiple foes.

Other problems post-Basil II were the decline of the theme system and the weakening of the Armenian and Georgian buffer states. In general the Byzantine Empire kept a tight leash on its military by controlling it with constant naval communications with the capital. Central and Eastern Anatolia however are far from the coast which allowed powerful families and tribes to eat up several themes and form private armies and independent principalities. A few of these went on to usurp the throne while others like the Armenians tried to band together and form their own kingdom after they had been settled in the region. So essentially the Anatolian magnates consolidated all the land that the thematic soldiers were supposed to have, thus weakening the civil and military administration in favor of local lords. The Byzantines tried to break these families and tribes up, and in the chaotic civil wars that followed bands of Turkic tribes came in and settled the dry central steppe. At first they were regular mercenaries in the in-fighting, and eventually attained offices and estates of their own under the Byzantines. But during the reign of Alexios Komnenos, when they and many others were rebelling, a Seljuk prince from the east fleeing an Iranian civil war began to unite the many Turkish beys in the region as a sultan, independent of the emperor's authority. The aridness and conditions of the central Anatolian plateau didn't favor agriculture like the rich coastal and western valleys, hence livestock raising was what the magnates pursued. This allowed the Turkoman raiders to bypass fortifications and settle on land that easily reminded them of their Central Asian homes. Once entrenched, the Byzantines couldn't easily eject them, because unlike the Arab armies of the Caliphates which depended on long supply lines which the Byzantine armies and border warriors could harass, the Turkomans and Seljuks were hardy steppe nomads. They could easily live off the plateau (and they also brought their vast herds of sheep and cattle) without a cumbersome logistics train. And it certainly didn't help that Konstantinos IX Monomachos managed to get into a massive losing war with the Pechenegs right before the Seljuks finished running over Mesopotamia and Persia like a freight train and Seljuk affiliated tribesmen started their incessant raiding into Anatolia.

Abusing the safety of the mountain passes and distance from the Byzantine dominated coastline, the Seljuks of Rum consolidated their position, and over the next few centuries nibbled away at Byzantine land when possible. This was a very gradual process. Even Manzikert didn't lead to the loss of the interior of Anatolia. The Seljuks stopped at Armenia and let the Byzantines fragment before gradually encroaching into the highlands over centuries. Even by the Komnenian period, they weren't significantly ingressed into Anatolia. Christianity was still predominant. Greek was still predominant. Most of the local villages merely kno-towed to the new rulers over changing their cultural heritage. The real pivot is the Battle of Myriokephalon. This more or less signaled that Byzantine hegemony in Anatolia was a thing of the past, and this lead to significant cultural changes in the indigenous Anatolian population.

Another thing to consider is that Byzantines got involved in clashes with Armenian and Georgian states that bordered eastern Anatolia prior to the Seljuk conquest. Those petty states would've been an invaluable buffer to absorb the shock of the Turkomans as well as provide invaluable soldiers to the Empire as they had in the past. Armenian king began losing power to nobles and power became decentralized, and the Byzantines took advantage by supporting various nobles. Afterwards, the Byzantines removed Armenia's 50,000 standing army so the buffer against the Seljuks was weakened.

Furthermore, the crisis of legitimacy after the death of Basil II and Konstantinos without male issue left the succession of emperors in the 11th century until the Komnenoi unable to trust their commanders and needing to spend the empire's increased revenues securing their support among the nobility and people of Constantinople. This was partly reversed by Konstantinos IX Monomachos, but patronage of scholarship combined with a decade long war against the Pechenegs forced him to debase the currency to keep the finances afloat. This would have taken a decade or two of relative peace to recover from, but instead the Normans and Seljuks left the empire in a constant state of war. Now, if they had a decade or so of peace after the Pecheneg War of Konstantinos Monomachos to get their currency back on a stable footing, they would have been in a much better spot to deal with the Seljuks and Normans in a way that didn't involve massive concessions to independent Turks in Anatolia. And we can only guess how things would've played out in that case.