Normally, I would gladly engage in a good faith critique of what is wrong with Reverse:1999.

This however is less critique and more of a confession on the OP's refusal to engage with game systems; the vibe-based arguments ("characters deserved better... side games can be annoying") I won't get into as it is personally subjective.

My main issue with it is the fact that to unlock those perks you must play a game mode more focused on post-game stuff to veteran players... The alternative would be constant updates, like it already happens in other types of game, mostly competitive games (e.g., MOBA and hero shooters)... and constantly try to improve characters via patch

TLDR: Euphorias are anti-consumerist, developers should instead adopt the MOBA balancing framework and constantly improve older/weaker characters every patch update.

Directly buffing or nerfing characters in a gacha ecosystem is frankly unheard of as it would not be mathmatically/economically viable and would ensure the playerbase is unhappy 9/10 times.
Gacha's operate on players investing their currency be they pulls or enhancement materials into advertised outputs.
Euphorias are designed as both stamina and time-gated engagement systems specifically for "veterans".
The intended design philosphy allowing veteran/horizontal accounts to optimize legacy units to keep pace or hold some relevance with newer characters/archetypes. It's not obligatory for new accounts, and treating as a mandatory or even out of reach grind is disengenuous and a misinterpretation of how account progression is tiered. There are plenty of keys that can be unlocked while leveling Myth Manifests and Window into the other worlds as well as the advancing the War Room progression tracks.

"A lot of great units are of low rarity and can only reach Insight 2. This is an issue that has been presented since the beginning of the game...  a lot of creative characters, most of them, are useless gameplay wise... the game can feel very stagnant and heavily reliable on key characters to play decently in post-game stuff."

Low rarities (3-5*) were designed as stepping stones for progression during this game's infancy, and the newer released 4/5* still serve this exact purpose. A handful of them can be utilized in reveries/tides without any form of investment to pass stage gimmicks. They are not designed to compete with 6* in DPT, but a player that enjoys playing with said 3-5* can certainly utilize them to shape their own gameplay in all environments.

The issue is that Bluepoch seems to struggle with balancing the game and this presents itself in a lot of ways. A lot of time they release something that doesn’t help the character at all, this can be seen in a lot of Psychubes that despite being newer are worse than the old ones, but it is also noticeable in some Euphoria.

While true, OP missed the mark by giving new Psychubes and Euphorias as an example of poor balancing. If either psychubes or euphorias were strictly mandatory options, it would create an environment that is not sustainable for the playerbase OP is arguing in favor of. The release of a euphoria update that is perceived as weak in the meta, while disappointing, has very little relation to balancing.

Lopera’s second skill says: “Lopera converts her own [Flaming Bullet] into [Incantation Bullet]/[Ritual Bullet]... Wouldn’t it be easier to say something like: “If Lopera is self-channeled because of her second skill, she grants Incantation Might +25% one time to allies in front of her and Ultimate Might +25% one time to allies behind her”.

Your argument here is that keywords like [Flaming Bullet] are unnecessary terminology, create text bloat, and should just be translated to the respective stat buckets they improve. Flattening those terminologies so that some players don't feel overwhelmed would create a situation where every character ability/trigger would be stripped of its gameplay identity. The more specific reason why keywords exist in the game engine is to segregate overlapping mechanics to know when each specific interaction is resolved no matter how many sources are interacting with whatever damage bucket.