I think it’s unfortunate that I’m the first person here with this attitude, but I LOVE the Lancer (gave me strong Windrunner vibes [Stormlight Archives]), which is an automatic win.
But I also think you are hitting the nail on the head with this. Prestige classes and synergy feats are awesome, but they don’t kick in until later in your class progression. I’ve noticed that a lot of people in these forums seem to think that doesn’t matter since the game theoretically goes until level 20, but it really DOES matter. People want to play their special, preferred concept from level 1, if possible, rather than spending weeks or months slogging through setup. I also think the type of people who seek out these forums are much more likely to have campaigns that run longer than most, not less, so aren’t representative of players as a whole.
I think this idea makes class design “easy” in the sense that yeah, you have new ideas and concepts in here, but it’s mostly just well-adapted alterations to Fighter and Adept, with a little Marshall in Sanguine Knot, adapted to the concept. It avoids many of the pitfalls of class bloat by keeping things short and sweet. And I think 8th level is a good jump-off point into other things.
Two personal preferences/ideas:
(1) I like ASIs, half-ASIs, and feats, and I personally would be drawn even more to motif classes if at 4th and 8th level you had the option to get an ASI. I know this is trying to fit in the design space of Prestige Classes and Synergy Feats, which gobble up your ASIs like candy, but I still think giving a +1 to an ability score plus one of the abilities you have above (at 4th/8th) is more than fair. Alternatively, maybe grant only one full ASI, but grant it late, at 6th/7th/8th. Just spitballing.
(2) Since this is like the reverse prestige class/synergy feat tree, coming before a class and serving as a jump-off into other things, I think there should be a note at the end how this class can be allowed to contribute to the classes that mixed to create it. I think this helps create a sense of continuity when multiclassing out of the motif class (if you want one) and allows you to build on certain abilities without feeling the need to build out the motif class to level 20. Some things are already built in here - with how multiclassing rules work with maneuvers, no need to mention that. But you could add, “8 levels of Lancer and 3/4/5 levels of Marshal or Fighter allow you to attack two additional times with Extra Attack,” or “8 levels of Lancer allows you to treat your adept die as one size higher when multiclassing into Adept, and 4/6/8 Adept levels increases your Lancer die to a d10.”
A LOT of the synergy feats/hybrid archetypes that I’ve been working on might be better as a motif class, so if you don’t mind I want to borrow the idea and tinker around with it.