My setting uses what you call free-form spell-casting.
I like it and my players like it. It uses a system and methods I developed myself, over time, with player input.
I allow players to use spells from gaming books if they desire to give them basic guidance on what they want to create, though after some practice the players usually abandon those books and start creating their own
"Spell-books" filled with ideas of exactly what they want to do and create. This also allows them to develop a very efficient method of generating individualized variations on already existing spells ad hoc. In this way how good a magic-user becomes really depends much upon their own imagination, innovation, and creativity, because effectiveness is not based so much on raw force (a weakness of many existing magical systems in my opinion, the idea that magic, like electromagnetism, is most effective when highest charged), as on subtle and clever manipulation of the forces, environment, and events surrounding the current circumstance they encounter. In this way magic went from being
"Hulk-Smash, jack up the current," to a more fluid and adaptable tool of surgical precision. (You can easily control how much force is applied.) Though occasionally the players still consult pre-generated spells from gaming books to give them ideas in a tight situation.
The same basic idea occurs with what many games call psionic powers (though I don't like that term, especially not for fantasy settings) in my world. So both magic and psionics are freeform. Ease of use comes over time. It just takes practice to master, as is the case with learning any system.
The one trouble I did have was in differentiating Clerical or Divine magic from Arcane magic, because if it's freeform then clerics might feel they have the right to create the same basic spells as Wizards, and vice versa. The best thing I did to solve that problem was to take away all spells from Clerics and instead center the Cleric and the Clerical type characters, Monks, Hermits, etc. on prayer, meditation and Miracles. Also with my system most Wizards have a very hard time healing themselves and others, and have an even harder time healing permanently, whereas most clerics don't most of the time. With free form casting though Wizards can become particularly effective illusionists, and clerics can become very potent focal points for miracles.
Miracles are kinda like arcane magic in one limited sense,
in effect, but are nothing like magic in the way they operate. Miracles in operation are almost totally out of the hands of the cleric (as far as direct control) but are instead in the hands of God. Clerics, Monks, Hermits, Saints, etc. can perform miracles but they cannot control what happens during the miracle or contour the manner by which the miracle will manifest. That effectively solved the free-form cleric-wizard overlap problem.
Anyways good luck to ya. With some experimentation I'm sure you can develop something that'll suit ya just fine.