To me, at first sounded like a broken (worn) Woodruff key.
But if ring gear turns, then balancer is connected to crank.
Also a little confused(maybe just the choice of words) .. you said "spanner and hit starter", which I assume you mean the large 27mm(1 1/16") bolt to remove balancer off crank. Spanner or socket and breaker bar.
Anyhow, after reading the last bit, sounds more like starter is spinning but not engaging, so it's spins, but it's clutch is not engaging onto the ring gear. Have you confirmed this either take the starter off or check ring gear whilst hitting starter?
Another alternative: you said "drive plate and ring gear" so can we assume an auto? You didn't say flywheel. Maybe cracked drive plate? Maybe still has just enough friction to turn when rotated slowly, but once starter is running at speed, just not enough and drive plate overcoming engine compression is harder compared crank turning the drive plate.
So if you have auto, most likely aircon too. Check the timing belt inspection port for timing belt happiness too.
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