The Saudis (as well as the other Gulf monarchies) are essentially on America's leash. They depend on America for security (especially against internal revolts). Here, I'm speaking of the royal families, not the country. Anybody who gets out of line meets the same fate as King Faisal (assassination). Below the king are a million princes with claims on the throne. There is a Hollywood movie called Syrianna which explains the situation very well.
China, I think, is not interested in these internal power struggles of Arabs, thus their influence is restricted to economics.
That's past tense, Saudia Arabia is officially in the BRICS in a shorter than two-week timeframe and is already in SCO for a few months (BRI too).
You can see how they officially welcome Putin, Xi, and other Russian officials and how they welcome Biden and American officials.
America can't provide security to its bases in the Middle East literally, look at what's happening to them in Syria and Iraq, how they are humiliated.
The US once had hundreds of thousands of troops in the Middle East, today they have a few tens of thousands, and in a way less welcoming environment.
America is objectively done with the Middle East. What security can they provide? Can they physically attack and destroy Iran?
China can provide better security by keeping communication between the Sunni and Shia worlds alive and influencing Iran.
China has to build its own military first for Taiwan, or the Saudis would probably also already have switched up the Chinese systems.
Because the US systems are majorly overpriced and they can't even arrive in a reasonable timeframe due to their small capacity.
I forgot to mention in my previous post that China is also the biggest creditor and partner for Saudi's 2030 vision and their country overall.
Public support polls in the Arab world already say that China has a lead over the US. The US can only threaten with hard power, nothing else.