What is the main purpose of this pc?
If it is for gaming, one of the 9th gen intel K suffix processors is as good as it gets.
A Z390 motherboard may be $200, a 2 x8gb kit of ddr4 ram might be $130.
For the processor, a $250 i5-9600K with 6 cores would be a very noticeable upgrade for you.
$400 might buy you an 8 core i7-9700K.
Any more than that for gaming does not get you more.
But $550 for a i9-9900k still is within your budget and might be appropriate for multithreaded batch apps also.
All three of the processors will run on boost or overclock near 5.0.
Add to your budget perhaps $85 for a very good cooler.
A Noctua NH-D15s will do just fine; I am not hot for AIO coolers.