It's a two-fold kind of question:
Is the scaling rounded up when the multiclass feats count as half our level?
And is shouldn't the advanced feat be available at 8th level instead of 6th if it's rounded down?
Or is intended that we get dedication > 1 or 2nd level class feat > 1 or 2nd level class feat from scaling > 4th level class feat, etc from scaling?
In D&D and Pathfinder, you generally round 1/2-fractions down. Like when you take half of 17 damage it is rounded down to 8 damage.
However, there is no rounding involved here. A 6th lever whatever with Rogue dedication counts as a 3rd level Rogue for the purpose of picking up Rogue feats via Advanced Trickery.
The point is that you can usually pick a single feat only once. Note that Advanced Trickery explicitly states it can be taken multiple times. So if you want, say, both Quickdraw and Minor Magic for example, you can't get both via Basic Trickery because you can take Basic Trickery only once, but can pick up your second choice with Advanced Trickery on 6th level.
|Some Kind of Chymist|
That's the gist of it. Of course, depending on what class you MC into, there may be other options available. For example, Sneak Attacker is available at lv. 4 as a MC feat, vying with Basic Trickery for your second dedication pick. For Cleric it's the choice between getting basic casting or a basic Cleric feat at 4th. You could use your 6th level feat to get both options in these examples.
But of course it means you are not getting what your main class has to offer, and that should always been a tough and well thought out choice.
Keep in mind you can’t take the Feat 4 that gives you a 1st or 2nd level class feat multiple times, and it is a pre-req. ie, you can’t take two 2nd level feats without the MC feat 4 and feat 6. True, at 6th level MC feat 6 is essentially MC feat 4, but now it scales with level and can be taken multiple times, and as noted is the only way to get two 2nd level feats.
Seems fine, confusing, but on closer inspection a fine way to setup the rule even if offering up the option of a 3rd level nonexistent class feat is odd.