You really don’t consider tests to be a problem, your software is stable, but a certification with specific requirements in terms of software testing is imminent? It's probably no consolation, but you should know you are not the only one facing this problem. Many opt for the tactic: "close your eyes and hope for the best". That is, the requirements are somehow met with a brutal show of strength; the main thing is that you will obtain in the end the desired certificate.
We believe that this can be done in a more elegant, and, above all, more efficient manner. You can see this entire process as an opportunity to improve the quality of your software.
This also applies if you want to achieve a certain Safety Integrity Level (SIL) or comply with standards such as IEC 61508, or derived standards such as ISO 26262, IEC 61511 or IEC 62304. Don’t be happy just with complying with a standard, improve your software!