There is no gap.
When viewing the NEP debate you also have to look at party succession, as we know, Lenin died in 1924.
What happened during the period was the NEP debates, which you seem to know so I'll skip. This came to a head in 1927/1928 during the procurements crisis. By this time, Stalin through his political skill and extremely important position as General Secretary had started putting in motion the plan which would boost him to power.
Most importantly, however, was Stalin's use of the "Urals-Siberial Method" during 1928 when he actively encouraged rural party officials to seized hoarded grain under a law from 1926 which prohibited all "ill intentioned" price rises in grain. Economically, the policy which Stalin would pursue is direct mobilisation through force.
Anyway, the result of the NEP debates was nothing, Stalin came to power
Hence, you must look at the NEP debates and the struggle succession as being intertwined with each other. Then after that comes all the stuff with Stalin such as modernisation/industrialisation/collectivisation and the rest...