"Of" has a tendency to be more specific, more targeted. For example, you might tell a friend "I thought of a book I think you might like to read." However, you wouldn't say "I thought about a book I think you might like to read" unless you actually did spend some time thinking about the book. If all you did was to remember the book and write down the name to tell your friend, then you did not think "about" the book.

these sentences have the same meaning.

It’s the same yes.

“i’m thinking about buying a bicycle” sounds more natural to me