Overall we try not to use language of 'Decline and Fall' anymore as it is irrevocably linked with Edward Gibbon and the thoroughly disproven prejudiced history of the 18th century he penned. To discuss entire civilisations as being in decline when they continue for four hundred further years is just too sweeping. If we were to use a similar time scale for England we would be unnecessarily linking the Hundred Years War (finished c.1470) to the English Civil War (c.1651) or the battle of Marathon (480 BCE) to the Roman takeover of Greece in the mid first century BCE.

Yes there is a diminishing of territory but those grand narratives of decline or even causation don't penetrate deep enough to historical reality. The Byzantines recovered from the Arab raids and were the most powerful state in the Mediterranean from 900 until c.1050. Again after Manzikert under John and Manuel Komnenos 1118-1180 the Byzantines regained all the wealthy areas of Anatolia and surrounded the Seljuks on all sides, all they failed to do was recapture the rugged Anatolian plateau that was easily held through ambushes and traps. In the main crusading period the Byzantines rebuilt their army, campaigned in Syria, Antioch, Cyprus and Cilicia and it was to them that the Crusader princes swore allegiance eventually during the 1160s. Amalric, the King of Jerusalem himself came to Constantinople to give homage to Manuel.

Arguably Myriokephalon in 1176 was the end to Byzantine dominance of Anatolia but equally the real finger has to point to the period after Manuel's death, the lack of a strong successor, the final revolution of Bulgaria and the fall of Constantinople to the Fourth Crusade. The latter was the largest catastrophe to ever occur to Byzantine civilisation, far more than 1453 as it shattered imperial legitimacy and unravelled the leadership of the empire even more so than the lack of a unanimous Basileus.

Regardless even 1204 wasn't the end of Anatolian dominance. The Nicaeans extended the Komnenian fortifications, decapitated the Seljuk sultan in combat at the battle of Antioch on the Meander in 1211. Personally, I always see the true collapse as a combination of the end of the Laskarid dynasty as it disenfranchised the Anatolian populations who had been rallied during the recovery, (Michael VIII Palaiologos blinded John Vatazes' son) and this escalated under increasing Mongol raids which shattered the Seljuks and led to the rise of the Ottomans. The last attempt to be the heads of Asia Minor was the Battle of Pelekanon in 1329 which lost Nicaea as well as Andronikos II's decision to outsource his entire naval strength to Genoa and Venice, disbanding his fleet in 1285 due to financial concerns. Those are the key dates I would suggest you keep in mind, they are the moments a maritime empire lost the ability to keep in contact and enforce governance over it's own possessions.

Looking earlier, even back to the Arab invasions for the decline of Byzantium as a whole just isn't historically valid. If you wanted to discuss the decline of Byzantine Egypt or individual provinces that fell to the Arabs of course, then that is a different situation.