The CPU can still hold up to an extent, the GPU being the weakest component in relative terms, however you can find yourself whether you would benefit from a CPU upgrade by checking the components utilization with a software like MSI Afterburner or OpenHardwareMonitor.
If, in the games you play, the CPU runs at 100% or displays a high-ish utilization, compared with less than full load on your GPU, then you are experiencing some degree of bottlenecking.
Also, this is more likely to happen at lower resolutions, such as Full HD, where the contribution of the GPU is diminished and more load is applied to the CPU. Weird as it may sound, the same game will rely more on the GPU and less on the CPU at higher resolutions, all else being equal.
Upgrading the CPU and RAM could still make sense if you can find components on the cheap (so, second hand and reasonably priced), an i7 could give some benefits in heavily threaded games, thanks to SMT increasing the number of virtual CPUs. You can also check how much memory pressure is being applied on the system while playing using appropriate tools, as this can indeed cause stuttering which is independent of the CPU’s and GPU’s own prowess.
The 1660 Ti is roughly a 2060 minus the RTX features, whose usefulness is ATM debatable. Both are more or less regarded as 1440p cards, so would offer some extra degree of future proofing at 1080, if not planning a monitor upgrade.
As for other suggestions, the price levels of SSDs have come down considerably over the course of 2018, so you’re roughly looking at a potential doubling of space at previous price levels.
That means you can buy a 500 GB unit for what your 250 GB may have costed, or a 1 TB unit if willing to spend some more.
Performance levels haven’t changed appreciably, as generally the SATA interface is the hold back; NVMe removes the bottleneck but increases the cost of the unit, as well a requiring a motherboard that supports that interface.
Still, 250 GB is small by today’s standards, so extra space makes sense as long as you don’t mind spending the money.