What you are missing is that skill dice cannot hurt you and are thus the most valuable dice in the game.
And additionally: Skill dice don't go away when you are injured!
Yes, while these statements are correct. They are kind of moot as this discussion is talents vs skills, and talents also lets you roll additional "skill" and artifact dice, that also cannot be damaged.
Trying to evaluate these:
A d8 is worth 3 d6 if you only count the average amount of "hits". (4/8 = 3/6)
A d8 is worth ~2.6 d6 if you if you only count your chances of scoring at least one "hit". (3/8 ≈ 1-(5/6)^2,57788)
Just for this argument, lets simplify and say that a d8 is worth around 2.75 regular d6 dice.
15 xp in a skill at zero gives you:
Each die then costs you: 7.5 xp.
30 xp in a skill at zero gives you:
Each die then costs you: 10 xp.
50 xp in a skill at zero gives you:
Each die then costs you: 12.5 xp.
18 xp in talents at zero gives you:
1d6 + 1d8 specialized skill dice. (Simplified as 3.75 dice)
Each die then costs you: 4.8 xp.
These dice are NOT as valuable as regular dice, their exact worth is up to the player.
But if worth lets say around half, which is kind of reasonable (as you also get a nice talent perk), then their cost would still be less than 10 xp.
So order of gaining things should probably be something like skill level to 1, then talent up to 3, and first then other talents or skill increases.
If you don't like that "everyone" always will go for talents first, then I actually think that the proposed house rule is the simplest way to solve this, you just need a list of what feats belong together with what skills.. and you should also tell this to your players before they create their characters, as this can severely mess up their character progress plans. This rule will also slow down character progress a bit, but that is probably only a good thing.
A second way is to change the cost of skills and talents.
A third way is to house rule that you loose the +1 modifier from talents when you receive the d8 die (up to you if this then also should apply to "horseback fighter" as they are different bonuses).