Slowly download large images over network to downscale it on client side is not the best idea. No, we can not scale in VNC client. Perhaps, VNC server can scale?
I don't use mac, but I saw screenshots with wtware connecting to mac via RDP. In RDP, client asks for screen resolution, so problem with resolution does not happen. Also, RDP is faster. Perhaps, you can try RDP?
VNC with 1080 screen displays error because of "only for testing" banner that uses 64 pixels. Write "give me one license to test full screen VNC that does not work with banner" to firstname.lastname@example.org
and we'll grant a license to test it without banner.