OP I think you're on the right track.
Older vehicles have their coolant recovery bottle at atmospheric pressure, and only drain into the radiator to top up as the engine cools and creates a small vacuum. You don't know it's low until morning after.
Modern vehicles have their coolant recovery bottle at system pressure, which reflects the coolant level. So that could work here.
I would say your scheme could flicker and false trigger due to sloshing. It's customary to have a long 30 second delay circuit to prevent this.
Luxury cars with a low coolant sensor, I have only seen a brass electrode in the radiator and even then, low coolant can still slosh and spray if the electrode is in the flow. It's a bit of a challenge because (in the cooling system) low coolant still surges, burps and splashes etc. My old Chevy temperature gauge would read fine when almost empty! Sensor was in the thermostat housing and I found low coolant meant the sensor never got wet... I had to move the temp sensor lower, into the cylinder head port (5.7L V8) and then it would finally read hot with low coolant.
The
WS-03 water sensor, looks it has an open-collector output which means it needs a pullup resistor to turn on a mosfet, same as what you are doing and like fish tank circuit here:
https://www.eevblog.com/forum/beginners/water-level-sensor-auto-to-up/ Same thing you are doing on a Fiesta, but no extra mosfet:
https://www.fiestafaction.com/threads/diy-15-low-coolant-light-no-hacking-required.56722/To your circuit, I would add a 2A fuse and series diode 1N4004 on power to prevent reverse spikes from damaging the sensor. 2N7000
K mosfet I use, it has extra surge protection built-in. BS170 but add a 15V gate zener protection. Or upsize to a TO-220 size mosfet. There's a lot of semiconductor shortages going on right now and the perfect part is not always available. If you want I can sketch the circuit.