I use mongodb, I found it is quick for prototyping. But I'm worried about race conditions, because I think its transaction is limited. But for my current stage

I use mongodb, I found it is quick for prototyping. But I'm worried about race conditions, because I think its transaction is limited. But for my current stage (MVP), it seems to be fine. I might eventually switch to postgresql, but I don't know how to deal with data migration once my database changes. I want the program design to be stabilized first. I will not use firebase, it will be very expensive. I have tried aws lambda once to create myself an email redirect service. I found it hard to debug.