I am using Chateau 4G as a router I take with me to trips frequently since there are no high-category routers in Mikrotik line that are smaller. Tethering my phone via usb to AC2 didn't work very well for me as reliability of that was pretty low (even though it was a Google Pixel phone), especially over longer times of continuous load.

The problem though is that with large antennas that it has, I have to pack it in the box every time, which takes a lot of space. Wondering if it's possible to use it without antennas by removing and disconnecting them, and possibly configuring the router differently in a way? Looked through product documentation and it doesn't even mention what these antennas are doing, and presumably they are wifi antennas.

I don't need extensive wifi coverage - typically it's a hotel room, and that's it. For critical applications such as multi-hour zoom calls I would connect ethernet cable sometimes just to reduce latency, which works very well, but some wifi coverage for casual phone usage would be nice of course.

So is it possible?

Don't know about Chateau but on AC3 and AX3 there is a very big yellow sticker on the device indicating NOT to use the wifi radios without antennas attached.

Not sure which model you exactly have but checked for Chateau 5G and there it says the antennas can be removed (when powered off).
So check your documentation again. It should be in there if it can or can not be done.