My daughter had a large AVSD as well. She had surgery at 12 weeks old to repair it. She's now 8 and thriving, and just under yearly reviews by the cardiac team.
I also found pregnancy incredibly hard, and spent most of the second half off work with anxiety. I wasn't allowed to take anti depressants (they decide on a case by case basis) but was under consultant care and the lovely mental health consultant said I could stay overnight whenever I felt I needed it.
Before we gave birth we were given a tour of NICU, and we were able to ask any questions we had.
My DD ended up in SCBU for 2 weeks, about 48 hours for oxygen and monitoring, and then the rest for feeding. She came home with an NG tube, which I was terrified about, but was actually fine.
I found the 2 weeks in SCBU quite helpful, it gave me time to come to terms with her needs, and to recover from the c section. Plus there's a community of parents in the same situation. We were given a Ronald McDonald room as well.
My main recommendations for you are to prioritise self-care during pregnancy, and prepare friends and family for the first months afterwards. You'll want to be really strict about people not visiting if they're unwell, as your baby will be vulnerable.
Heart surgery was stressful, but there's so much support in hospital which helps. My mum basically gave up her life for the first 3 months to support me, which was amazing.
Also, do learn to ask for help from friends and families, and if they offer, give them specific tasks. We had help with things like getting shopping in, picking my daughter up from nursery, even doing our washing! It made so much difference, and they feel like they're helping as well.
I wish you and your baby all the very best.