I was also undiagnosed with both pregnancies. I didn't find the body changes too difficult, but the first birth was traumatic (failed induction resulting in EMCS - there were other complicating factors in my 'care', too). It took me a while to acknowledge how much it affected me and now I'm having trauma therapy for PTSD.
However, DC2 was an ELCS and the sheer control I felt over that delivery was so much better - I knew when, how and that I'd be out in 24h barring complications. I would definitely give that serious consideration in advance. Likewise how you feel about internal examinations and being touched by lots of people if you need monitoring, etc. You can refuse internals so it's good to think in advance. Also think about having someone with you all the time to advocate if you're getting overwhelmed.
The things I've struggled most with post-birth are the lack of time to myself, constantly being available to deal with kids who need physical contact, never getting to properly relax. One DC is also autistic which requires a lot of extra thinking for me. When they are upset by routine changes or sensory overload I'm dealing with it too, for both of us, but there's never enough downtime to decompress.
The various changes in routine, breastfeeding, changing needs, returning to work, trying to carve out space for myself, not having slept properly in years is now taking a significant toll on my mental health. Had I known at the point you are what I know now I'm not sure I would have made the same choices (this is no reflection on my DC who I love beyond anything) but a reflection on what this process has done to my ability to cope with the basics of life. It's almost impossible to mask now. I can't keep up. I feel overwhelmed and have more meltdowns which I have to hide from others. My relationship and my work are suffering, I feel I'm failing in all aspects of adulting.
Read around as much as you can. Look at where the uncertainties are, from my experience: conception isn't guaranteed, pregnancies don't always work out, there's no way to know what birth will be like, you don't know how your baby will feed or sleep, you don't know if they'll be autistic too (with all the sleep deprivation and support they might need), etc. Look at what mitigation you can put in for each step so you're as prepared as possible.
I'm sorry mine is a cautionary tale, maybe once I'm through therapy I'll find it easier to focus on the positives (and there are many of those, despite the way this post reads). You also have an advantage as you already have a diagnosis, you won't be trying to cope with pregnancy and motherhood and all the attendant physical, routine and sensory changes whilst not understanding what's happening and why you're struggling when others appear fine.
I think you're wise to consider this in advance and wish you luck on your journey.