Buffing her R for 0/5/10 damage per sphere is the laziest and worst direction you can take for her, and as a 1.5 millions points Syndra main i assure you there is no need for a discussion here.
Her main problems are:
- Transcendent has nothing to offer for the entirety of the early game.
- W's damage and slow are too low for how hard to hit/easy to miss the skillshot itself is. Maxing it also gives us worse damage than maxing E until rank 5 since it has lower base.
Since you want to keep it low, just tweak her W numbers for it to be more rewarding to hit and max, something like "90-145-190-235-280 + 80%" and make its Transcendent bonus be "add 10% scaling and convert 20% of the spell damage to true damage". This way we'll get a stronger W, no R changes and the W buffs you planned for her now instead.