"The tidal-locking also suggests that the planet may not still have a spinning core."
The opposite's the case. If you're phase locked, then your core gets tidally heated. So there should be enough energy to keep the dynamo ticking over. That's why Mercury (3:2 resonance) has a magnetic field. It's likely TOI 7000 d does as well.
It's the atmospheric effects that will be the killer.