i7 10700KA, it is the same as the K but is generally 1-5% cheaper and comes in a faggy Avengers box with a code for the game. Overclocked with adequate cooling it can match an i9 10900. Get an RTX3080, best card value. Get whatever mobo you feel like getting, preferably one with an integrated sound card if you give a fuck about that kind of stuff. I generally go a full ATX build because there's literally no reason to go smaller. You can go mATX if you want a slightly smaller board and don't intend on using all your PCIE slots (i.e if you don't want a 3090, Titan, or don't intend on using Crossfire/SLI). Water cooler system is like $100 for a decent drop in. You can, despite what some believe, achieve LC performance with a traditional heatsink/fan but the coolers that put that kind of performance up are more expensive than the LC kits. Don't bother filling RAM slots 3/4 if you don't intend on graphic design or something extremely RAM intensive. Due to a combination of hardware, software, and driver limitations virtually no games can actually pull from those slots, so if your main focus is gaming it is best to get a 2x16GB (or 2x32GB if you aren't a faggot) RAM set and just leave slots 3/4 open. If you do decide to use a smaller board, it won't matter because it will come with only 2 RAM slots anyway.
Any questions I can answer for you?
Edit: AMD is shit and there is nothing redeeming about their CPUs or GPUs. If you really are on a budget, get an i5 9600K. It is $200 and performs better than AMDs 3900X in every gaming benchmark. The i5 9600K is objectively the best budget CPU for gaming. With GPUs, there is literally no reason to not buy at the minimum a 3070. I cannot stress this enough, the RTX 3000 line is the biggest breakthrough in GPUs in two decades. It's massive. I know it sounds gay and is more than you want to spend, but buy a fucking 3070 at the bare minimum. By benchmarks the 3080 is technically the best value, but for like, half the MSRP, you can get 2080ti performance with the 3070. So that's my """budget""" recommendation: an i5 9600K (OC'd with water cooling) and a 3070.