Can I make just my bedroom a smart room instead of the whole home?

Yes, of course, the smart home is scalable. This means you can
start with a small device or start with your bedroom. Once you are
happy then you can think of expanding the smart home benefits
based on your budget and time.

Alternatively, you can try lighting automation of the entire home
alone initially, and later you can do security automation, kitchen
safety automation, etc. step by step to match your need.