To the OP.
You usually place about one, or sometimes 2, full divisions lower than where you ended in the preseason.
So going from Silver 2 to Bronze is not out of the ordinary. I was in Gold V at the end of season 5 and placed in Bronze I or II. Climbed through silver pretty easily to get back into Gold. I expect, when I start playing ranked when the season starts, even though I finished Gold II I will probably place in low silver.
I will say, you play too many champions probably to effectively climb, and your consistency isn't there. You have some good games, but you also have some bad ones.
Your builds need some work too, but maybe you are just trying new stuff because it is preseason. My guess is that you don't effectively push your lead when you are ahead with map pressure and objectives.
I climbed 3 accounts through silver last season, my main and my two smurfs. My main account placed low, my smurf that I created to play and stay in silver ended up getting up to Gold IV or III. I used it to just play a warm up game or with friends in silver and ended up winning too much.
My secondary smurf I created just to practice a new role and new champion and it ended up getting to gold V.
If the skill is there you eventually float to your rank from playing.