Don't quote me on this but as far as I know, playing ranked throughout the preseason is not a waste of time. At the beginning of each season you are reset to unranked and have to play 10 promos to set a new rank to climb from. Where you end up after those 10 games is a combination of how many you win (win as many as possible) and some equation riot made that puts the average mmr, last seasons mmr, preseason mmr into a big melting pot and spits out your new rank.
Last season I ended Plat 3, won 8/10 promos and got gold 1 as a starting point. Dont worry about placements though, if you made it to plat and can now play like one then even if placed in silver it will be very easy to climb. I got something like 23-24 lp per game won and lost like 13-15 lp per game until I got back to my proper rating. Good luck in ranked!