As you mentioned, you certainly don't want to pad the length. Don't beat yourself up about meeting your 50k goal. Some people suggest that 80k constitutes as a novel length; others suggest that 40k is more appropriate. It varies drastically depending on who you ask! That being said, if you feel as though the story is finished and you reached a natural end based off of your outline, then this is the perfect time to start revisions. As you said, your novel will go through multiple drafts; it's an incredibly laborious process, so you might find yourself tinkering with scenes, deleting chapters, rewriting chapters, and adding scenes to places you might find are noticeably scant. If you notice the early chapters are a little thin and missing critical components (e.g., exposition, the framework for future character development, etc., etc.), then it's completely fine to add more as long as you find it critical to the story as a whole.
If you're concerned about what's missing or what you should add, I think it's pretty useful to take your eyes off your first draft for a couple of weeks (maybe even months!). It sounds crazy, I know, but once you look at your draft with a fresh pair of eyes, then you can truly see where the draft's pitfalls lie. I always find when I finish something or continuously tinker with my draft, it's super easy to ignore my mistakes altogether. I've also found with the students I've tutored in creative writing, they find it useful to take a break on their full-length project and maybe move onto smaller writing exercises and prompts to distance themselves from their first draft for a little bit before revisiting it again with fresh eyes. If you take a break from editing, I'm sure you'll notice where you need to make necessary additions. Hopefully, this helps!