Where can I find a Sephardi siddur with none of the Arizal's influences - essentially, a Pre-Ari Sephardi Siddur?
I would look for a British or Dutch Sephardi siddur. After Sabbatai Sbi, the S&P community actively removed all Lurianic and Zoharic emendations to their nusach (source: R' Jonathan Cohen of the London S&P minhagim site)
You could start with the Rambam's סדר תפילות כל השנה found at the end of משנה תורה - ספר אהבה.
The Rambam predates the Arizal by over 300 years.