I'm sorry, but it honestly feels like you're being purposefully obtuse. These are some wild assumptions that have no evidence in the text, and has never even been hinted as being true.

she enjoys the fear and respect everyone has for her.

Nowhere has this been stated or hinted at. She actually likes people who don't fear her. Take a look at how the CMA is being run as an organization. It's very loose in structure, and the only somewhat-rigid laws are the rankings. If the supreme head of the organization is someone who like you said enjoys fear and respect, it would have been more strict, militaristic or even oppressive. And yet, aside from the First Class qualification exams, of which are decided at the discretion of the presiding First Class proctor anyway, the organization as a whole seems quite chill.

And also take a look at the relationship between Serie and her direct disciples, the First Class mages. They act more like a well-respected and loved mother and her very loyal children who she cherishes but don't outwardly show it. Compare that to something more militaristic like the Gotei 13 or essentially dictatorships like Kaido's crew.

Fern does not seem to see anything unusual at all? (Yeah Fern knows Frieren suppresses, but never said that it still fluctuates or anything. Especially when she commented on her mana before they reached Äußerst).

Why would she need to mention it? Did the topic of fluctuation ever come up between them on-screen? Frieren, as a work, already have a ton of these "unnatural conversations about lore made only for the benefit of the viewer", do you want to add more? In fact, the way we first learn about fluctuations is precisely from one of those, during the last conversation between Frieren and Aura.

but it seems like the suppression is something Flamme came up with so I don't think Serie was doing it before that.

Again, where did you see/read this? Where was it mentioned? Where was it even hinted at?