In point of fact this all very simple. We certainly aren't gaining anything by going on about it for 6 pages.
All you have to do, understand that intent of the rules is that you only observe it when it is cast, as it has always been.
I'm curious about what makes you the authority on what the intent of the writers is. Things have changed before as to how they worked in-universe from one edition to the next, how do you know this isn't another such example?
It's not complicated it doesn't require new elaborate complex house rules to keep a long list of spell as they were intended. Just understand that the perception of magic rule are only intended to be used when the spell is cast. Just like the Book says it is.
Really it is that easy.
I can accept that we have a fundamental difference of opinion about whether the rules
should mean what they say. I have to agree that you may even have a point that perhaps sustained spells aren't supposed to be perceptible, but this has gone on for 6 pages is because according to the book they are.
The book says that all magic is perceptible. Prove me wrong: reference is on pg 280, 1st paragraph 1st and 2nd sentences.
The book says that some magic is SO perceptible, that there's no need to even make a perception check. Reference: inferred in 1st paragraph, explicitly confirmed in 3rd paragraph.
The book says that of the magic that is subtle enough to require a perception check, you do it in one of two ways. Reference: 2nd Paragraph, 1st sentence.
The book gives an example of a spell being cast for the first mechanic (Skill-F) and a Ward being encountered for the second mechanic (6-F): Reference Second Paragraph, sentences 2 and 3.
There's no room to debate any of this so far without citing a 5th edition source that exempts sustained spells.
There's no indication that spells can only be observed during the moment of casting in the 5th edition rules. Prove me wrong: cite something that says otherwise. Again "how it was in prior editions" is flimsy and can be dismissed simply by saying "yeah, but that's how it was, not how it is now".
The example of wards being relevant for the 6-F mechanic is actually very important. You must make a skill check to "perform" the Ward's casting, but yet according to the example to support the rules you don't make a Skill-F threshold check. The examples are also not said to be exhaustive/the only cases of what qualifies for the given tests. The language used for leading in to the examples is "For example", which unambiguously infers that other circumstance can also apply to the forthcoming examples.
Since you "perform" a skill to make a ward, and yet you use 6-F to perceive when an observed later, and examples aren't exhaustive lists of when to apply the rules, "the rulebook is saying" that you make a 6-F threshold test when an observer is exposed to magic post-performance. This implicit statement wouldn't be there had the example for F-6 been perceiving a Spirit lurking in astral.
Again, don't conflate opinion and even prior editions with what the 5th ed rulebook is unambiguously saying.