Stating that diversity of thought existed in the effectively autocephalous European Church from the 5th through the mid-10th century conflates accidental diversity with intentional. It’s true that the Church was more decentralized in this period, but this diversity arose from a lack of centralized control, not from a deliberate policy of tolerance or openness.
Before the Gregorian Reform in the 11th century, the Papacy’s direct influence on 'frontier' archbishoprics was relatively negligible. However, as the centralization of Rome, the Roman Rite, and ecclesiastical subordination to the Papacy took full form, the Church’s suppression of divergent practices and beliefs became systematic. This period saw the emergence of many of the 'truly bad' features often associated with the Church, such as uniform enforcement of orthodoxy and suppression of dissent. See R. I. Moore's The Formation of the Persecuting Society for a full treatment.
Your argument is like claiming that Red Hat in the 2000s promoted diversity because independent Linux distributions existed. In reality, Red Hat itself has always been monolithic. The existence of an independent Linux ecosystem that preceded Red Hat does not make Red Hat a promoter of diversity. Similarly, the diversity of regional practices in early medieval Christendom was not a feature of the Church but a byproduct of decentralization, which later reforms actively sought to eliminate. Diversity arising from a lack of control is not the same as cultivating it within an institution.
Moreover, reliance on orthodoxy is a feature, not a bug, of the Roman Catholic Church. This reliance has historically been central to the Church's identity and governance. Attempts to portray the Church as inherently diverse prior to the Counter-Reformation overlook its systematic suppression of autochthonous rites and regional practices as centralization took hold. Rather than prevaricating, Catholics need to embrace this reliance on orthodoxy as central to the Church’s unity instead of this kind of revisionism.