I reached out to the author of fasmg WRT your post and circular dependency interest and he pointed me toward two posts that he wrote very specifically to explain what he believes is unique to fasm/fasmg and allows to handle circular dependencies of many kinds. [0] Types of multi-pass assembly, and [1] related pitfalls.
Thank you so much! It looks like the issue I described is what he calls the 'oscillator problem' [0]. This is an absolute goldmine, I'm going to be reading for days.
[0] https://board.flatassembler.net/topic.php?p=178828#178828