I can confidently state that the P515 is not compatible with my specific BB386 frame (Felt VR '23, the old one, BB is token ninja TF38624), at least not without drive side spacer shenanigans.

Regular installation just ends in the cranks locking up fully once the bearing pretension is set by feel, the beveled face on the back of the spider collides with the very bottom edge of the frame.
After bodging a 3d printed spacer somewhere around 0.7mm, i didn't measure the actual thickness, i'm getting a gap i can stick 0.65 feeler gauges into and a 10km test ride went without anything feeling or sounding off.
I'll check for scratch marks when i get my hands on proper shims and adjust accordingly.

Obviously i'm not happy with the situation but i don't feel like admitting defeat either.

Bought it from 'EU Tax-free Discount Store' which went fine, arrived in seven days from france via fedex, balance and pedaling smoothness features were already unlocked.
Box showed signs of having been opened before, my assumption is that it is a chinese SKU and the seller activated the spider prior to shipping since the magene app did not have any activation prompt and just paired readily.

PS: excuse the account new-ness, i've been lurking for a couple years