First of all, High and Late Middle Ages is when “self-expression and self-determination” really became a thing.

There were medieval scholars in early (“Dark”) middle ages who wrote about self-determination in the context of a greater community as part of the development of Christian intellectualism. I would read this part here, but the whole article is quite interesting (sites.nd.edu/…/moral-self-determination-and-the-b…):

The most well-known literary source providing an exposition of obedience is The Ladder of Divine Ascent, authored by John of Sinai (c.579–659 AD).[3] In the fourth chapter or “step,” John addresses the practice, defining it thusly: “Obedience is absolute renunciation of our own life, clearly expressed in our bodily actions…Obedience is the tomb of the will and the resurrection of humility.”[4] His endorsement of the renunciation of “will” may sound odd to many readers, especially given the Christian emphasis upon moral self-governance. **Nevertheless, John is not denying the concept of free will as such, nor is he suggesting that the volitional faculty must atrophy into non-existence. **Scholarly evidence suggests that the term John uses here for “will,” thelēma or thelēsis, comes to be associated with the volitional faculty in a philosophical sense in the writings of Maximus the Confessor, whose engagement with the Christological controversies of the seventh century provided the impetus for the standardization of the expression.[5] Thus, when John speaks of “will” and its denial, he is arguably referring to what Maximus the Confessor and his theological progeny would call gnomē, which in the idiom of the time refers to a private or particular disposition of will, or even to a personal opinion.[6] John’s monk is not so much denying his own intrinsic freedom of will as he is seeking the co-governance and insight of those who are more advanced in virtue, and, through them, struggling to direct his volitional disposition such that it harmonizes with the other members of the community.

The idea being that one should self-determine, but also then be humble enough to know one’s limitations and understand how to harmonize your will with that of the community. The preceding paragraph really brings this idea home:

Maximus discloses a similar approach to moral self-determination by establishing his ethical teaching on “love” or agapē, which figures prominently in his philosophical and dogmatic treatises as well as his ascetic writings.[7] Agapē is no mere private sentiment but constitutes the impetus and ground for moral practice as a whole, thereby suggesting that moral judgment and orientation presuppose an awareness of one’s community and the persistent presence of a real, tangible “other.” In this way, Maximus retools an older Aristotelian paradigm, exchanging justice for love as the central and all-defining virtue.[8] Insofar as agapē is the chief virtue, narcissistic self-love, or filautia, is its inverse and the progenitor of all vice. As he demonstrates in one of his earliest works, The Ascetic Life, ascetic discipline should not be considered a private enterprise intended primarily for the sake of internal moral perfection.[9] Rather, its purpose is the effacement of filautia and the diachronic restoration of temporal and eternal relationships with the creator and one’s fellow creatures. To quote the Confessor directly: “He who is unable to separate himself from the passionate yearning for material things shall neither love God nor his neighbor authentically.”[10]

I am not a proponent of using religious influence to guide one’s morality or decision making, but I am just using the above paragraphs to discuss your first point.

Second, oldest democracies formed before those ended by any criterion.

You’re right that the history of democracy and democratic societies predates Medieval history, but historical examples of Western governing systems in which middle classes could participate are more well-known in the middle ages

The first parliamentary bodies involving representatives of the urban middle class were summoned in 12th century Spain. In 1187, the Leonese King Alfonso IX summoned representatives of the nobility, the church, and representatives of the 50 most important cities, to a council in San Esteban de Gormaz, Soria. There was another meeting with representatives of the cities in Carrión de los Condes, Palencia, the next year, which institutionalized the Curiae.[23] There had been other meetings previously, such as the Concilium of 1135, but they were exceptional and not leading to a regular attendance of town representatives. According to the UNESCO Memory of the World Programme, this is the earliest documented manifestation of the European parliamentary system with some temporal continuity.[2][24]

source: en.wikipedia.org/wiki/History_of_parliamentarism#…

Essentially, people sought a centralization of power so they’d have an easier time dealing with the governing bodies–“one king and his court” vs. many nobles. Here’s a nice summary: en.wikipedia.org/wiki/Parliamentary_system

Third, a typical modern centralist democracy making citizens equal is hostile to self-expression and self-determination, for the same reason any centralist state is.

By definition, there is no self-determination under the rule of a cult leader or authoritarian as you’re subject to define yourself by their will. The democratic tradition, in its various flavors, tends to lend some leeway in enabling anyone to exert their opinion and shape the way the community thinks. In fact, this tech dude wouldn’t be able to spout off his nonsense without a democracy of some sort, which is why we’re unfortunately exposed to his gibberish and now having this discussion.

Fourth, medieval societies became a relic of the past because they couldn’t scale as easily as modern ones in terms of state bureaucracy, and thus manpower and firepower.

Because the rise of parliamentarism (a type of democracy) helped form more efficient governing bodies.

[…] what they called honor and chivalry were pretty specific things, and not “everything good, kind, holy and manly merged”.

I know :) The point I was making, however, is that people seek some greater purpose or meaning to align their will with that of others.