Safari 14.0 no longer uses flash. First question is the Mojave that most recent update of 10.14.6? Is there a reason you are not using Catalina 10.15.6 such as 32 bit apps? It was against the last such version that Safari 14.0 for Mojave and Catalina were tested against. 🙂
The new Safari is definitely wonky. Even in Catalina, it hiccups when opening, and quitting it takes longer than previously. I suspect it might be due to the new tracking features.