Wow looks like the Zeng are certainly no pushovers. Constantinople better hope they don't get too close to the Ottomans
A united China never is really.
What is to everyone else's benefit is that current policy seems to be very weak in naval concerns. This will severely hamper their ability to project power.
For one, someone else will need to fight the naval wars. Against the Romans I'd expect Spain, and vice versa.
This means that the most likely situation - for China to assist the Ottomans, it will be a logistical nightmare and one that could shatter the Zeng if they overcommit - and I expect at the hands of the Khazars, Tibet and Ship Lords.
An army to ensure the Ottomans can pull a D3 on Constantinople will need to be similarly huge, and I doubt the Ottomans can really supply it. Even taking a huge amount of time to prepare supply depots, a supply chain from China Proper is insanely vulnerable to the Khazars. That means they'd have to survive on local supply or pillaging. That'll wreck the region. I'm not even convinced that if secure the overland route has the capacity.
Which leads us to the sea.
They have no window on sea, Oman, Ethiopia, the Romans and Ship Lords are all in the way. You'd need A Spanish-Triune domination to secure that. Sure a change in policy and developing a strong navy could flip that, but it doesn't look like that's happening. Even if that did happen there is Japan on the mix too who shouldn't be overlooked.
The Zeng at the moment might be best placed as a regional soft power that you don't want to face on land locally. But anyone expecting them to send a big enough army to save the Ottomans in the next war is going to be disappointed I expect.
Personally though I'm all for a glorious attempt by the Zeng, only for the new Khazar King to show he's got his predecessors skill and crushes the Zeng supply system and teaches them that they can't project out west militarily. Not without providing a literal slave army to the Khazars.