However, there was probably also a symbiotic / empathatic relationship which occurred after the Ring was physically removed from him by Isildur, so that when Sauron began to realise that his power was beginning to grow, he would have realised that it was the ring that was giving him strength even though there was no physical connection.

The ring gave Sauron a greater focus on mind control at the expense of mobility - although he would still need to be in a physical form. Sauron could still suggest and influence but not dominate unless he had the Ring, or the Seven & Nine for men & dwarves only respectively (which would explain the need to keep the Seven and Nine in his possession).