I am not sure what you mean by "Immortal playthrough"?It is true that this can give an immortal character the ability to attain truly phenomenal skill levels, but then again, the whole "lifestyle" system is inherently balanced by your lifespan... Immortality breaks things.

Regarding commander traits: Interesting. I like this idea! Definitely adding this to the list.

Well for my immortal game i have mod that decreases skill experience so there in not so easy to get all perks. What i mean is more like life experience where the character gets just better doing things. Like are smarter to survive plots, lead armys, learn new languages. And some mods add kind of different bonuses to whole dynasty like resistance to plots or adnatage in battles, but i think it will break game in longterm. So it would make sense that character who has lived trought life should have thouse bonuses. Even if he(she is not immortal just living long life with aquired game bonuses, like books and health perk tree. Modifer that allows to you to develop and get ahead others. But thats just opinion after all thats your mod to do as you wish.

"just living long life with aquired game bonuses"
I believe this is the point of this mod. You get better at what you do over time.
That's why I added the ability to improve education traits in parallel to "lifestyle XP".
This is also why I added the Councillor's traits and the courtier's traits. All of these are meant to represent people getting better with time/experience.

I've added your idea for improving commander traits to my "roadmap". I also want to create sort of "hybrid bonuses". Things which will reward you for breath as well as depth of expirience.
[just for example, a crafty priest with high intrigue should be better at forging claims. Or a diplomatic steward should be better at spreading culture.]