Yes, the UDM is quite deep and heavy. If your rack also has rails at the rear, then you can also install equipment racks such as this Intertech equipment rack for server racks Max. Regarding the idea with the switch: I prefer to leave one U of air between the devices, especially if the rack is not actively ventilated.

Yes, the UDM is rather heavy. I installed the UDM in my rack without a device carrier, but I still have to tighten all four screws at the front of the mounting quite firmly. However, the UDM does not sag in my rack. As a rule, I always leave one U of space between the devices so that the air can circulate better or I can attach a panel with cable guides. I have a rack from 19Power.de with 15U and a glass door.