I had a somewhat similar system in mind, but the like others have stated, Materia needs to remain as an effective gear and gilsink, and making a materia-melding class diminishes the value of the original crafting classes.

Still, I think you can still accomplish this with a hybrid of the current system and your suggestions, along with the suggestions I've seen from others on the forum. We can offload some of the randomness of x-melds to a combined random and progress-based spiritbond system. For example:

Melding
1. Gear cannot be destroyed when melding. Each meld is a 100% guarantee.

2. A piece of equipment will have a certain number of slots, up to 5. "Better"/rarer/+1 gear for its level will have more slots. Slots with materia in them can be overwritten, but the materia occupying the slot is destroyed.

3. Melding still requires the use of catalysts.

Spiritbonding
1. Materia is only created from spiritbonding and converting gear.

2. Materia that is created from converting gear will range in tiers (I~IV) and potency (+1~+20, etc.), as it does in the current system. Higher tiers and higher potency are adjusted to be noticeably rarer than they are in the current system; let's say 10~20% rarer as a placeholder.

3. The type and total potency of Materia attached the equipment you are trying to convert influences the the type, tier, and potency of the resulting materia. For example, a player who converts a pair of Felt Bracers with three Strength II materia will have a higher chance of converting bracers into a higher potency Strength III or IV materia.

In summary, what this tries to accomplish:

1. Preserve materia as a gil and gear sink by maintaining the incentive to acquire and convert gear.
2. Remove randomness from melding gear.
3. Increase value of +1 gear.
4. Increase value and usability of lower tier/potency materia.
5. Increase value and usability of lower level gear or gear with lesser quality melds while still providing an incentive to convert said gear into materia.
6. Could, with just the slightest chance in hell, make Status Resistance melds easier to make, and as a result, make said melds more desirable.


TL;DR: Remove randomness from melds, increase gear-to-materia conversion rates by incentivizing melding of lower tier materia onto gear, do so by allowing melded gear to convert into better materia.