Yes I remember chatting Dan, although I forget if we chatted about your CPU though. Having only 4 threads will hit framerate as a whole depending on the game As you can see in the video below (using a Vega 56 with a Ryzen 2200g and Ryzen 2600 - The 2200g is faster than a 2500k despite it's lower clockspeed). This was my upgrade path. I actually thought I had a faulty Vega with the fps I was getting when I initially paired it with a 2200g but I replaced it with a 2600 and FPS have pretty much doubled in some open world games
It depends on the game but as you can see with Kingdom Come Deliverance it bottlenecks heavily at medium settings. If I remember rightly even with the settings amped up settings the CPU bottlenecks it heavily.
Also if you have limited threads leaving browsers and other programs running in the background will see you get worse performance than in the video. I was actually getting lows in the 30s-40s with Just Cause 3, which never happened after the upgrade
Looking at these videos the 2600k should perform quite well once overclocked, the problem is I'd like to see him use more open world benchmarks because it is likely to suffer with these
As you can see in this hardware unboxed video - the 2600k will perform great in racing games but will suffer in Assassin's Creed, Shadow of the Tomb Raider and Hitman when scenes get busier and Kingdom Come Deliverance will similarly be hit
So you will still be CPU bottlenecked in some games and it'll be more noticable in busy areas of open world games. But in racing games you'll barely notice the difference.
In this video you're comparable CPU is a 2400g, which is a little faster but similarly has 4 cores/8 threads. So I'd imagine the performance upgrade should be similar to the 2200g to 2400g comparisons below
As for long term replacement I'd recomend holding on until Ryzen 2700 goes end of life and drops to around £100 (happened with the 1700) or wait until the 4000 series release when the 3600 probably drops to around that price. Both will give you a nice upgrade in open world games for around £240 for CPU/Ram/Mobo
One takeaway is if you upgraded to 1440p instead of getting a new CPU/system you'd likely suffer no FPS hit