One thing I found crucial when dealing with estimation is to always, always explicitly add the level of confidence (in the estimate). This saves so many tough discussions later... either have it built in to your product development process (e.g. cone of uncertainty, or different stages with different predefined levels of confidence), or always communicate it right next to your estimates. Some stakeholders will push back initially, but you can reason and tell them that you need to invest more time into research, planning, PoCs etc. if they want a more accurate estimation. Eventually, it boils down to "are we agile yet", trust in the organization, maturity and culture. One of the questions I always ask when interviewing for a role is how this is done in the company, with actual examples when projects took longer than planned (and what happened). Tells a lot about the maturity of the org.