Advertising without working measurement is spending budget on gut feeling.
If every sign-up counts as one, the algorithms optimise for volume. Pass on what a sign-up is worth on average, and they steer on revenue. That is usually the biggest improvement you can make without extra budget.
A pixel that is both in the code and in GTM counts everything twice. Results then look too good and you raise a budget that does not deliver. Checking beforehand prevents months of steering in the wrong direction.
Some of your visitors refuse tracking. That is their right, and it means your numbers are incomplete by definition. Work with ratios and trends, not absolute numbers — see also server-side.
A question or project on this topic?