If the problem is that your manager isn't listening to your estimates, then find a new manager. Plenty of fish in the sea.
That's why I often 3x my estimates. It works great for me.
It's never a problem that I finish a project early. But when external launch plans are on the line, it can definitely be a problem to finish a project late.
Finishing a project on time is one of the things you get evaluated on in performance reviews as a software engineer. It's also up to the software engineer to provide reasonable estimates and to push back on requirements or incorrect estimates coming from higher up.