I'm kind of worried that 1200W+ is running through the traces in the board. At the same time I wonder if it's possible to push 8x Vega VII at 250W each

Bought one of these similar boards as sample today, but has a cpu socket to use a G4560 instead of the soldered 3865U. It's slightly cheaper than the onda boards and most importantly $86 USD Bitmain APW7 can be used instead of 1500w titaniums.
If on Windows..... I may share the same concerns with you... that's why putting this on Linux SMOS with tighter power controls is preferred without messing with Windows, registry, power-tables etc.
The 2400w Deltas are rated 94% platinums - I have got a bunch of these (during Parallel Miner firesales). They are really a live saver and I dont have to worry about number of cables needed even for heavy duty GPUs like 1080tis or 32 cables....
you do not need to setup with all that power into the mobo
psu send 6 pcie to mobo
psu send 8 pcie to cards
do not feed a lot of cables out of the mobo into the cards.
less energy goes into the mobo.
somewhere some place I did photos on this board and
sending cables right to the cards = better
then cables to mobos to cards
the mobo still needs 6 cables into it for the slots to be powered.
it does not need any out of it into the cards.
although it is neater wiring .