Hotends need insulation. Not just to prevent melting the mounting bracket, but:
To prevent heat loss to the air, especially with part cooling fans active, that could reduce the ability to heat or even trigger thermal runaway protection from complete inability to increase temperature beyond a point.
To prevent dumping heat on the printed part, which fights with your part cooling fan and greatly diminishes its effectiveness.
At the very least, the heater block needs a silicone sock. You can also use high temperature ceramic fiber or "rock wool" insulation like you'd use for an oven or kiln, and/or just layers of aluminum foil (which reflects a lot of the heat, while air pockets between layers provide insulation).
To find an appropriate sock for your hotend, search for its model name together with "silicone sock". You can also make your own using high-temperature RTV silicone (for example Permatex 81878) and a mold if you have a block with odd dimensions that nobody seems to be selling socks for.
In your question you mentioned "all metal". In the context of hotends, "all metal" generally doesn't mean that there is no non-metal insulation or other material attached or that it's not okay to attach such, just that there is no PTFE or other material that can't handle high temperatures in direct contact with the hot parts. It's perfectly reasonable (and necessary) to put proper insulation on an all-metal hotend.