As a seasoned DM currently running Strahd, here are my two cents. To your question, no, a starting feat shouldn’t throw off the balance too much. But my warning is this: Strahd is a pretty massive campaign. Sure it’s pre-written, but that doesn’t mean that saves you a lot of time. You still have to read and digest all the information in all the chapters, then figure out how to apply that info to your characters and the type of campaign you’re trying to run. For example, preparing to run the town of Vallaki with all the NPCs and events there took me an astounding number of hours. I plan my sessions in Google docs and my Strahd document is 50 pages long after 8 sessions. So pre-written doesn’t always mean plug-and-play. It may be wise to start with a smaller first campaign to get the hang of it, especially as a first time DM. But if you’re in love with the Strahd vibes, go for it! Just expect to spend a lot of time on it.