I am intrigued by the effect die concept, and love the tier system how about combine them so that gear is not the thing that determines effect die size but skill tier.
Example Diplomacy check d20+ability modifier vs DC if successful roll tier effect die and and ability modifier as well.
Untrained = effect is ability modifier only
Novice= d4+ability modifier
Journeyman= d6+ability modifier
Expert= d8+ability modifier
Master= d10+ability modifier
Grand Master= d12+ability modifier
You could make only three tiers at d4, d8, and d12 to simplify things.
Keep the idea of higher tiers open up new uses of the skill, that those not at that ability can not even try.
Have the right tool for the right job like masterwork thieves tools give advantage (+2 to the d20 ability roll).
This way the tier does not change the ability modifier check but the how effective a success is.
Not sure if this is a good idea, but think it is worth contemplating.
Example Diplomacy check d20+ability modifier vs DC if successful roll tier effect die and and ability modifier as well.
Untrained = effect is ability modifier only
Novice= d4+ability modifier
Journeyman= d6+ability modifier
Expert= d8+ability modifier
Master= d10+ability modifier
Grand Master= d12+ability modifier
You could make only three tiers at d4, d8, and d12 to simplify things.
Keep the idea of higher tiers open up new uses of the skill, that those not at that ability can not even try.
Have the right tool for the right job like masterwork thieves tools give advantage (+2 to the d20 ability roll).
This way the tier does not change the ability modifier check but the how effective a success is.
Not sure if this is a good idea, but think it is worth contemplating.