Unfortunately, when it comes to analytics and conversion pixels, deferring everything isn't really a serious option because it literally means missing data. Essentially all your metrics would be skewed and your remarketing audiences would be missing potential users. But also from performance it isn't great - I came across this website yesterday that did more or less this, and the problem was that because everything was loaded just after the page was ready, things would get super slow the first few seconds you're trying to interact with it.