I'm breaking out the 0.8mm pitch BGA of a STM32G474VE using a dog-bone fanout and a 1.5 oz outer layer (so, need 6 mil trace/space widths if possible). To facilitate this, is a land diameter of 0.35mm for each ball ok? The datasheet recommends 0.4mm.

[TDK]

That's a better question for your board house and in-house design rules, rather than ST.