The reading order for this series is a little less straightforward than most, which seems to cause a lot of confusion. You definitely start with the main novel, 'Nevertheless'. After that, there's often a debate. I'd go with the direct sequel, 'Nevertheless: Aftermath', before diving into the prequel 'Nevertheless: Prelude'. The prequel adds a lot of context that hits harder if you already know where the characters end up.
There's also a companion collection of short stories, 'Nevertheless: Interludes', which you can really slot in anywhere after the first book. I scattered them in between the bigger entries on a second read-through, and it enriched the experience, but reading them as a final 'epilogue' works fine too. The publishing order is a mess because 'Prelude' came out last, but narrative-wise, this order makes the emotional beats land correctly.
Just skip the so-called 'side story' fan compilation unless you're a completionist. It's not by the original author and feels off-tonally.