How much space do I need between rafter ties?
"...ceiling joists are not supporting the roof in anyway, they are just tying the walls together"

Unless you have a big ridge beam you failed to mention, this is a brilliant piece of misdirection. While superficially correct, what do you suppose would happen if the walls were not tied together? The roof would collapse under it's own weight! No snowload required.

The lateral force exerted on the walls from a low pitch roof is substantial. While it is possible to stabilize the roof with collar ties alone, the end connections become very critical in low pitch applications. You'd be much better off with a high pitch roof in terms of spreading forces from the roof.

If you're going to do a major structural change anyway, I would install a ridge beam along with proper end supports in order to achieve maximum openness from my efforts. Regardless of which approach you select, for any major structural change you should consult with an engineer to ensure everything is done safely. Small details can make a big difference.