It is probably cause by no neutral wire on the smart switch?

The Broadlink  TC2 apparently was designed not to use a neutral line. I.e. one to one replacement with normal malaysian switch.  Works with higher wattage bulbs perfectly but just not LED bulbs below 7.5W.

That is because it uses the circuit as series, sure it maybe ok for large some range of voltage and current. Do you know such switch will create voltage drop, and some sensitive devices will have problem, also for large current operation device may result in burning the switch.

It is not much different because the wire is really not that long. Cut it short is to make it more proper and have some space for the hot air out. (not much also)

You won't get risk of fire if

1) the time using is short.
2) The heater KW doesn't exit the max wire ampere carry. Maybe around 20A. or 4.8kw.
3) the connection not loose and tight secure. switch and heater side.

Get a better higher Amp switch.

I know.

That's why I told him, not much different. But trim down the wire in a tight space will have the wire properly done and some air space. A tight space behind the socket will bend the wire 90degree at the screw contact wire side. With the thick wire usually use in the wall, the wire have intend to brick at the end insulation expose wire side.

You know all this consumer wall socket even a 20A socket don't have a cable lug type of terminal. So you only can tightly screw the wire to get a secure contact.

I usually will strip the insulation longer, bend the wire to equal length and insert into the hole. That increase the wire contact amount in the hole. It also help for the smaller wire gauge that the screw have difficult to lock it.

I use a lot of cable lug in industry. If the consumer socket, plug do have, definitely I will advise them to use it.

i tot wires making a small loop carrying high current will act like a heater element?
better not to loop the wire inside the switch box. make a clean connection to reduce the induced current