My plan, for semi-pro work, video editing etc. was to go for the R7 1700 now, then likely go for Threadripper2 on 7nm in late 2018-early 2019.
8 cores for sub-£300 seems like the true sweet spot. The 1600 6-core is the best for spending as little as possible, but the extra 2 cores on the 1700 is incredible value as well and gives you some breathing room.
Then Threadripper2 looks like it'll be 16-24 cores, instead of 12-16 cores. And each core will have mildly higher IPC, and clock to 4.5+ GHz due to the GloFo/IBM 7nmLP process.
By the time more than 8 cores is truly needed, Threadripper2 will be out and a ridiculous upgrade from the R7 1700, where the R7 1700 was a ridiculous upgrade from Sandy/Ivy Bridge.
The most up to date benchmarks I could find for Adobe Premiere Pro show it doesn't scale past 10-cores, for example.