PX4 doesn’t really “support” MAVROS ROS2 setups. They should work but it will be harder for you to get support.
I only have general PX4 comments on arming and offboard mode. The requirements for arming in offboard mode, or switching to offboard mode when armed are listed here - angular velocity, attitude, and offboard signal.
What that means is that you have to supply velocity and attitude, and stream a proof of life message. That means meet the same requirements as are met in Offboard Mode (Generic/All Frames) | PX4 Guide (main).
If you were using MAVLInk directly you’d just stream the message that set your desired target, and that would do the proof of life and your target in one.
Hope that helps.