As to whether you should present your findings before, during, or after publication of the full research paper in a journal, that largely depends on your goals for giving the presentation and consideration of publication ethics.
Before publication: If it’s more important to disseminate your research as quickly as possible (e.g., to establish priority in some technological fields or when your findings are of immediate benefit), you may consider presenting before submitting your research paper to a journal. This would also be useful for fine-tuning your study or full research paper based on feedback from attendees. Usually journals do not consider poster presentations as prior publications, but check what constitutes prior publication in the guidelines of any journals that you may want to submit your full paper to. In any case, it is good publication practice to inform the journal editor of any previous presentation, in whole or in part, of the paper’s contents when you are submitting your paper.
During or after publication: Presenting a poster about your published paper can be a great way to get ideas about future directions of research. It can also be instrumental in expanding your study’s reach so that others become aware of your research. This is helpful not only because people may change their thinking/actions based on your work, but also because it may increase the chances of others citing your work and thus increase its impact, which is important for career development. However, note that you need to consider publication ethics, and specifically copyright rules, carefully in this case. As explained in detailed advice from the Committee on Publication Ethics (COPE), check the journal’s guidelines and your publishing agreement about whether this is allowed and in which form. Moreover, check the conference guidelines as to whether the conference allows already published work (or forthcoming articles accepted for) to be presented, because some conferences do not.