I, personally, do something of a mix of what you are suggesting. It requires a bit of negotiation between the player and GM, but I think it leads to some interesting spells and sorcery.

Essentially it boils down to the magician player stating what they want to do spell-wise and talking about the requirements for the spell with the GM as well as what magnitude. While it does take a bit longer than just allowing the player free-reign it helps to create an atmosphere of sorcery being weird and truly arcane.

As I said, it takes a bit longer and is something I'd only use in my home games with players who I know are on board. In a one-shot or convention setting I'd just keep it loose or offer up example spells (like the ones listed in the book) as a frame of reference for the player.

IN swords and sorcery, most of the time the caster does something new and interesting each time they cast. Free form permits this and should be encouraged. I do like themed casters so implementing that may be what you like. Use the lower of the difficulty if in line with the casters theme and the higher if not. You could also then deem anything opposed to the theme as impossible. For example, a caster who embraces the flames  could easily cast fire based magics and cast other types with a bit of difficulty, but never do anything with cold or water.

I do like the new spells that G-Man has in most of his adventures.   Learning them as a specific spell might proved one requirement in the right circumstances.  (Like if you ahve the Tome they are in...)