How does ground close the circuit, why isn't the current miniscule?
Body resistance varies depending on the path and quality of contact (dry or wet, size of contact, etc.) with the voltage. It’s in the multi-kohm range, so not so high that direct contact with 230V won’t possibly be fatal.
Ground, by definition, has practically no resistance. In practice electrical systems take great pains to minimize their resistance to ground, including driving large copper spikes into the earth to form a good low-resistance connection. The power panel ground will have a resistance to earth 25 ohms or less if it meets codes.
You’re also overlooking a detail: with AC, the current will find its way through any capacitance present as well. This means that,  even if you are insulated from ground by say, wearing rubber boots, touching live AC will charge and discharge through your body to the dielectric that’s doing the insulating. So even with those rubber boots you’ll still receive a shock, though not as much as if you were standing on the ground with bare feet.
This is why power poles use large stand-off distances for the high-voltage wires. This decreases the capacitance as well as increasing the insulation resistance. This is done for safety as well as reducing line losses.
Your ‘levitation’ case is the classic bird on a wire: the bird assumes the potential of the wire with no direct path to ground, and a very large insulation distance, so very little current flows through the bird.
Related: Professor said no current flows to ground