Lessons learned, missing one

So last year for the first time i grew Tomatoes in the Netherlands in Europe. Got myself some good Tomatoes from a store, harvested the seeds and went from there. Now of course i made a lot of mistakes, this year i will do better... Just, i made sauce from the (frozen) Tomatoes, and i was a bit disappointed with the taste. So, my question is, what can i improve for more flavor? Leaving them longer in the plant maybe? Not freezing them? Let them ripen longer? Please share your thoughts about this.