Cool rigs, the watercooled one looks really good.
There are also these chinese PSUs with 8 PCI-E connectors, can't comment on their reliability however: https://www.aliexpress.com/item/32869823311.html
And there are breakout boards that include an ATX connector but they are pretty expensive: https://www.parallelminer.com/product/zsx-breakout-board-16-ports-pico-24pin-atx-fan-hubs-100-gpu-mining-a-game-changer/
But the Seasonic should totally be fine for 3 1070s unless the board really requires all risers to be powered, which would seem stupid but who knows? But even then you could use Molex-4-Pin->PCI-E adapters. Would be a shame to let that quite expensive PSU go unused...