There is not an easy answer here.
You have a relatively well balanced system.
My guess is that you will benefit from more threads than you have now(4)
That is because you get problems while multitasking.
Is 8 enough?
Do you need 6/8/12/16 or more?
It is hard to tell.
Some games are graphics limited like fast action shooters.
Others are cpu core speed limited like strategy, sims, and mmo.
Multiplayer tends to like many threads.
What is your budget?
I think the simple thing to do is to buy a used I7-7700K which will set you back some $250-$280.
You will not be able to overclock it with your B250 motherboard, but the stock and turbo speeds of the 7700K are higher than the non overclockable i7-7700.
If it turns out that you need lots more, then you are looking at bigger bucks for a later gen ryzen or 8th gen intel processor and motherboard. I say go used because the resale value will not change much if you need to resell it.
There is another potential issue, and that is ram.
When you multiprocess, you need to keep all the active tasks resident in ram.
That may require more than the 8gb you have.
Since your motherboard has only two ram slots, you are looking at replacing your 8gb with a 2 x 8gb kit.
If you originally bought a single 8gb stick, you are running now in slower single channel mode.
Adding another 8gb stick will be good if it works.
But, Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, particularly AMD can be very sensitive to this.
This is more difficult when more sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
Then, on graphics. If you need a stronger graphics card, your 450w psu is not going to support a significantly stronger graphics card.
Bottom line is that unless you have the budget for a big rebuild, I suggest you go slow and do a simple cpu upgrade with a 7700K.