Active-Finish126's journey

I recently accepted a job offer from Company A after a lengthy interview process that started back in May. They took almost three months to finalize everything, and during that time, I faced a lot of delays and communication issues. They pressured me to make a decision quickly, so I accepted the role, even though I was still in talks with other companies. Now, they have pushed my start date back due to visa processing, which has given me some time to reflect. I just received news from Company B, where I am currently going through background checks. I am much more interested in this position because it offers better pay and doesn't require me to relocate, unlike Company A, which also didn't provide relocation assistance. I'm feeling conflicted about how to handle the situation with Company A since I don't want to burn any bridges. I know I need to reject their offer, but I'm unsure how to approach it after already accepting. I want to wait until I have a firm offer from Company B before I inform Company A, but I also want to be respectful and professional in my communication.