But I see you have an HCF4001 with 2 unused gates.......................
So you could use those to buffer pin 15 from your stove signal. Then when the D1 Mini is in reset, the pull down on pin 15 should do its job.
I have a modified schematic but I can't seems to add it to the post!
Disconnect D1 mini pin 15 from the diode and transistor base resistor.
Connect pin 15 of D1 mini to 4001 pin 12 & 13.
Connect 4001 pin 11 to 4001 pin 8 & 9
Connect 4001 pin 10 to the diode and transistor base resistor.
So the Tx signal on pin 15 of D1 Mini goes through the first gate of 4001, then through the second gate then to the diode and base resistor.
The 4001 gates are 2 input NOR gates. With the 2 inputs connected together, the gate becomes an inverter.
2 inverters in series means the signal is not inverted but is buffered