first of all if you only have 100k pageviews a month you're probably not even making $599 from serving ads
and second i don't understand what Zapier is supposed to accomplish in your mind?
maybe you have been using it in a way I can't think of?
from what I think you are talking about: is zapier supposed to like sit between website & like some SSP or DSP? during the bid!?!?! or just to like establish an audience to target like send your new email based id to a DMP or wherever for a customer who wants a PMP deal or something?
Also I'm almost positive Segment in your example is not used in the way you describe to pass user/device identifiers between ad platforms? Like i think they connect to adwords but only to grab the standard reporting data you can get into bq anyways. not any user or device data (impression level stuff is now complicated in their own odd like 'private' data clearing house thing).
TTD Unified ID would be what you're looking for