I would recommend KB 1066x card over 1200x (buy 256gb cards not 128gb, they are slower for some reason). I can fill up my cards @ 1080 25p with Hdmi out. However it's very important to set the canon settings right to make this work.
You need to maximise the cameras free buffer memory and watch out for settings that use too much processing power.
Photo settings should be raw
or smallest jpg only! <-- One of the most important setting to get right.
No auto white balance!
Auto focus off on af lenses
ML settings
Card needs to be Xfat and larger than 4 gig files enabled.
Some card warmup is recommended. I also recommend rolling a few seconds if you have shut down the camera because there seems to be a bug that sometimes makes the first recording stop after just a few seconds.
Be careful with too many ml functions enabled. I do use global draw on but I turn peaking and zebra off. I guess you can get cont. rec with these on but if you don't need them it might increase your cont. Rec stability.
Hdmi out, if you don't need it is also something that can be turned off to increase stability. I always use a viewfinder so for me it's always on.
You can also try Raw instead of Mlv and GL draw off for recording.
Deleting clips on card in camera to reserve space while recording can lead to frame drops/stopped recording when you continue to use that card.
There might be more settings to tweek that I just don't remember right now
