Victory Road ought to offer a good deal of exp, since even the wild Pokemon are all around 46 and there's a good deal of trainers, so catching up shouldn't be a problem.
Cynthia will definitely be a challenge, given her lowest levelled Pokemon are still level 58 and her team is incredibly diverse. Doesn't make things any easier that she leads off with a Spiritomb back during a time when it had no weaknesses.
55 was my go too level range for the E4 in Gen 3 & 4. Cynthias biggest threat to me was her Garchomp since I used to Nuzlocke Platinum all the time and I wasn’t always so lucky to have a Pokémon that can learn an ice move.
Have you taken the opportunity to catch the lake guardians yet? You need to register all the Sinnoh Pokemon in your dex in order to unlock the national dex and the post-game areas associated with it (at least in D/P. Can't remember if Platinum changed this.)
No not yet. I just got to the Battle Frontier and most things are blocked off. I have Rotom already but haven't seen Manaphy yet. Thanks for the heads up.