HB, much of what you propose would require an engine rewrite of epic proportions
.
Actually, myself, I think lvl 4 sites are too hard to find already. Basically, you only find them with spells, since it is never worth the time of an actual lvl 4 mage (with a bare few exceptions, annointed of rhuax maybe since they're guaranteed an earth) to wander around looking for stuff.
At that reate, a lvl 5 site might as well be lvl 9. Both options are supported under the current mod script, by the way - you can also add a bunch of common sites that make extra gems, if you think the current mix is income poor.