I was in China a couple months ago and ExpressVPN worked fine for me, are they actually using their own custom protocol?
When I was in China last April, SSTP worked fine (though long-lived connections tend to become slower over time, and then need a few minutes of cool down before being usable again). Most Chinese people I met were using shadowsocks.
But in a vlog I've watched on Youtube, the host of that vlog said "Over the last coupe of days, ALL the VPN is been very difficult to use". So, I guess that includes ExpressVPN.
Here is the video if you interested: https://www.youtube.com/watch?v=EuEdYvQmVFg (5:20)