As a first look I did a white and black build(had started before I saw your 'black build', so thought might as well put up anyway
Put Phanteks case in as is a good case on offer at the moment, and they're throwing in vertical gpu mount and riser cables as well if you want to go that route
The Asus board is a quality B660, priced at the top end of B660 but more budget end of Z690. It's a full ATX board but does have a pcie gen5 gpu slot and can take 3 gen4 nvme's so on a par with z690 anyway
Put a 12600k cpu, mid range, but still outperforms my 8 core 5800X cpu. You could opt for a 12700 or 12700F, OCuk don't sell them, put around £330ish
32 gb of no frill RAM 3200C16
Put a solid gen4 nvme ssd in there 1TB. If you need more seen the 2tb SN770 for £175
Arctic liquid freezer II is a no frills aio which offers best performance to price..another option if you want white to do black/white contrast with some lights would be the Corsair H100i elite capellix which seen on offer for £100
Also put a 1000W PSU in as reasonably priced, but this can be changed. This is total overkill for a RTX3070, but seeing as this has 10year warranty, will see you though next gpu upgrade, and next gen are more power hungry...and was only a little bit more that some of the 850W models...however have seen a Corsair RM850 for £90 which is more than enough for this build also, saving £40
You could drop to a B660M Mortar also for £160. It's a micro board and very good also, gpu slot is gen4 rather than gen 5, and think only has 2 nvme slots. Budget but good is the MSI pro B660m-a board but up to £150 so prefer the mortar at that price
Other option is a MSI Pro Z690-a wifi board, but saw for £225 on OCuk which think is too expensive...if you don't need wifi, the non wifi is about £170 I think, which is another good option
My basket at OcUK:
Total: £961.90 (includes delivery: £0.00)
Below is the h100i on offer for £100 for a black and white theme
My basket at OcUK:
Total: £139.99 (includes delivery: £0.00)
other options for black case
My basket at OcUK:
Total: £496.86 (includes delivery: £0.00)
other motherboard option..don't know if you require wifi or not
My basket at OcUK:
Total: £1,154.86 (includes delivery: £0.00)
you could also go with an air cooler for £40 for a12600k, 12700 I'd go aio myself