That's why you need the Form C reed switch similar to the one I linked earlier.
Form A reed switches have a pair of magnetic contacts hermetically sealed in a glass tube with a small gap between them. When you apply a magnetic field, the contacts are attracted to each other and when the field is strong enough to overcome their springyness they touch, making the circuit. Form C reed switches are a little different. They have an additional non-magnetic contact at one end, and the end with the two contacts has stubbier stiffer ones. The moving contact from the other end is longer, and sits in-between the fixed contacts, and is held against the non-magnetic contact by its own springyness.
You can operate a reed switch with a permanent magnet or with a coil, typically wound round it. Use lots of turns of fine wire to make a reed relay controlled by its coil voltage or a few turns of heavy wire to make a current controlled relay.
A form A (SPST-NO) reed switch is the commonest type, but as you need to interrupt power to the main relay coil when the load current is excessive, you need the form C (SPDT) type so you can use its moving contact and NC contact to open the main relay coil circuit when the load current exceeds the desired threshold.
Wind the heavy wire for the current coil round a dowel, round rod or cross-point screwdriver shaft a little thicker than the reed relay body, as the reed switch is glass and rather fragile and its far too easy to snap or crush it if you handle it roughly. When bending reed switch leads, to avoid chipping the glass seal or breaking the glass, *ALWAYS* hold the lead between the desired bend position and the glass body with pliers, not touching the glass, and bend the free end of the lead against the pliers without touching the other leads or the glass body.
Reed switches are not precision devices so you'll need to experiment with the number of turns to get the desired trip current. Also the position of the coil around the reed switch has a considerable effect so you can use that to fine trim the trip current. When you have the reed switch and current coil correctly adjusted, lock them together with a little hotglue - you can always use a hot air gun to soften the hotglue if you need to remove or adjust the coil.
To protect the reed switch contacts against arcing you should use a RC snubber across the main relay coil 100R and 0.1uF should do nicely unless the main relay is a contactor or other heavy duty brute with massive coil inductance. In that case, use an anti-parallel diode that can carry the main relay coil current + a resistor directly in series with the diode, equal to the DC coil resistance of the relay coil. Don't leave out the resistor - as a relay with an anti-parallel diode directly across its coil is slower to release - not what you want from an over-current trip.
The combo of a read switch current sensor and a contactor or heavy duty relay to break the circuit is obviously not going to be as reliable or accurate as a proper DC rated circuit breaker, nor does it offer the anti-surge delay characteristic typical of a thermal or magneto-termal circuit breaker. Another DIYable option, would be an electronic 'fuse' circuit - an analog current sensor feeding a comparator to determine if current is over the desired threshold, triggering a flip-flop to remove the gate drive from a high-side P-MOSFET power switch. Some automotive high side power switch chips have a current sense output so all you need to add is the comparator circuit and the flip-flop.