Anyone know how I can connect my Samsung Galaxy S5 to my Sony AX 20000 to use as external monitor ?

I have been filming wedding receptions ( dancing ) with the camera on my shoulder. However this puts the monitor about 1 inch from from my face. Although it allows me to move around the dance floor with smooth footage. Also makes holding the camera for longer times easier. I am not a fan of monopods as I usually end up with jerky transitions when moving around.

Although, I will still need a way to mount it on the camera at comfortable viewing distance.

I know a shoulder mount camera would be better but I like the form factor of my hand held camera other times.

not really possible (last time i checked for tablet compatibility) there is no HDMI in and any solution would have considerable lag (ie adapting HDMI signal to USB 2.0 and then using OTG on-the-go dongle adapter)

for your situation (been there, i hate wedding work), i would grab a cheap cheese plate type adapter (something like the wooden camera cage), a ballhead adapter, and a cheap portable field monitor that can do 1:1, frame with the BMPCC and watch critical focus and whatever misc data you want on the field monitor.

ive done just that several times for run and gun stuff.