Do add "in my opinion" or prefix with "I think", because your definite wording implied you were stating a verifiable fact. Telling opinions like they are facts and then backtracking with "oh but it was just my opinion" is a big problem in (online?) society / discourse, and has led to a lot of misinformation and anti-scientific takes spreading.
"The earth is flat" - "Can you prove it?" - "Oh it's just my opinion". It's dishonest.