#3844
Ngl, that's some weird shit.
Why would you buy a two years old motherboard that costs more than the CPU based on the same old 6.5 years old architecture that Intel finally moved away from? You could dig an old 7700K out from somewhere and it'd be faster and much cheaper. 7700 (and 6700 for that matter) would be slightly lower clocked but you'd still be able to find those for a fraction of the price.
You need either a budget as an upwards limit or a performance goal as a downwards limit, and generally try to make it a little less weird. Either you want a new CPU or you don't.
#3847
There is no 5600, only the 5600X. It's much better than a 3600 and the 3600 is old enough that it's getting hard to find a new one, especially if you want it to actually be cheaper than a 5600X. So I'd say the 3600 is right out.
12400 is a bit faster than a 5600X in pretty much everything and cheaper, though the mobos are more expensive, so the price roughly evens out. No idea how it works out in terms of performance with overclocking though, because the 12400 can sort of be overclocked, but you need a rather expensive mobo and Intel will probably try to get that locked down again asap.
If it weren't for mobo prices and Intel incessant need to ensure their customers don't get anything they specifically paid for, it'd be easy. As it is, you genuinely have a choice where there's no clear winner and no real wrong answer, unlike in recent years.