GTX970 is a bit faster than a 1050Ti, so not sure where that is coming from.
Trying to prevent a bottleneck is somewhat pointless. Either you computer can handle a game engine well enough so that it doesn't matter, or it won't. The GPU will have idle time if the CPU isn't fast enough. On the other hand, anytime it is not, you can crank the graphics up as high as you want and get better quality.
The idea of bottlenecking is under ideal circumstances. It makes no sense to put an RTX2080Ti in there and then try to run 1080p at 400FPS, it just won't work. You take that same system and want to run 4K 120hz, then the 2080Ti might be fine with that level of CPU. It all comes down to the processing tasks the CPU has to do. Now that probably wouldn't work either, but it just demonstrates what people mean by bottleneck.
Every system has a bottleneck, when building it is best to avoid making anyone too much that you have wasted potential that you paid for.
In this case, you, presumably, want to get the most out of the system that you can. Putting the fastest GPU you can afford in there now is a somewhat wise investment. As long as you plan to replace the CPU/Motherboard/Ram in the near future.
In fact that might be the better option for you. Instead of picking up a used GTX970, which are about $100, and some DDR3 ram, you could put a little more into it and get something like a Ryzen 2200G, B450 motherboard, and 8GB of DDR4.
PCPartPicker Part List
CPU: AMD Ryzen 3 2200G 3.5 GHz Quad-Core Processor ($79.99 @ Amazon)
Motherboard: Gigabyte B450M DS3H Micro ATX AM4 Motherboard ($64.89 @ OutletPC)
Memory: Team Vulcan 8 GB (2 x 4 GB) DDR4-3000 Memory ($41.89 @ OutletPC)
Total: $186.77
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-08-02 14:58 EDT-0400
Then start saving for a decent GPU, and possibly even future CPU upgrade.