Isn't the outcome the same, if passive is the floor? It seems like your choice on that is cornering you into modifying the feat.
Not quite. Setting the passive (with the +5 from Observant) as a floor for an active check has a very different statistical distribution than simply granting advantage on the active check. If I set the passive as a floor, then players would always prefer active checks to passive, and my goal is to make the choice of active/passive not substantially affect the outcome.
For reference, I always roll versus passives on my end--I never compare a passive score to a fixed DC--so the distributions of (e.g.) a trap rolling against a passive+5 or a player rolling with advantage versus a static DC are kinda/sorta equivalent. (Technically the passive+5 is still advantageous, but for typical combinations of bonus vs DC, the odds are pretty close.) The one place that doesn't work is an active Search action vs the original Dex (Stealth) check, but in that particular case the rules effectively impose the passive as a floor anyway, since the original Dex (Stealth) check to become hidden would have failed if it didn't beat the character's passive.
Are you saying here - we don't want some traps that need Investigation, and some that do not? If so, that was the kind of consideration that just lead me to say it is consistently Investigation for artifacts.
I'm fine with some traps requiring investigation, and others not. I simply don't want to feel like I need to force any of the former into the game just to make Investigation worthwhile. And since Gather Information and Research come up pretty frequently in my games (they're common player-driven methods to get a leg up on the opposition), I needed a proficiency for them anyway, and Investigation is a great thematic fit.
A bad way to rule in my view is where Perception lets a character notice say some fine seams in the wall, but they would still need their Investigation to know they signified a secret door and how to open it. Either compounding the difficulty, or halving it, depending on if both must succeed, or success with either does the work. After many sessions I landed on just saying - it's Investigation for an architectural feature like a secret door because even if you did notice the seams, it was still going to use that skill at some point. I suppose one imagines the character noticing there is an inexplicable void between rooms, and putting that together with other suspicious features... or whatever.
I agree sequential perception and investigation checks are usually a bad idea. The (rare) sorts of traps where investigation would come into play at my table are set-pieces to be interacted with, rather than mere obstacles. (After all, if someone went to the trouble of designing and building a trap with a mechanism complicated enough to be difficult to understand, presumably it does something important.)
My starting point was more as some other posters rule... but it just created inconsistencies for me in play. It's interesting how often I come back to consistency as a measure of a good ruling.
I too am all in favor of consistency, which is why I put so much emphasis trying to make sure that both active and passive checks have similar chances of success. That way I can just pick the resolution method that seems best at the time.
