I agree with OP. Cosemtics DLC is fine, even though I still feel a bit milked when I pay 20 bucks for an expansion, which does not include the cosmetic stuff that gets released along side it. But I get it; it doesn't add anything to the game but cosmetics, they are not very expensive. They are optional without missing out gameplay wise when not buying them. I think it's petty, but I'm okay with that.
Selling Focus trees as DLC is a big step beyond that in my opinion. The difference in the options you get, which have a big impact on gameplay, seem pretty major when using the Poland one, as an example, compared to the generic one.
Paradox introduced legion of DLC with all their games since CK2. For the most part they have walked the line pretty well, regarding what things to release as expansions, and separate DLC. Ideally, I would have liked the focus tree to get filled in over the years for free in the patches. An acceptable compromise for me would have been to fill in the generic focus trees over the years as part of the big expansions. Selling them mostly as separate DLC or bundled with cosmetics, I don;t want, crosses the line for me, personally, to start feeling like I'm trying to be milked for my pennies, and that HoI4, more than any other Pdox game before it, has been made as a vessel to earn many times again the price of the game by selling ungodly amounts of DLC, both cosmetic DLC, and parts of the game mechanics.
I find having only 8 focus trees, Which are a central feature of the game, a bit light, seeing it is the deciding factor in the distinction between the nations you play. It is a central part of the game they will most likely will try to make us pay dearly for. It just feels wrong to me, and a big step backwards compared to what Paradox used to do.