In skimming through this, if I were in your shoes, I'd not bother trading a 300WSM in on Rem's big 7... the difference is too little.
I'd also not consider bobbing the barrel on a magnum much if at all, as they need that longer pipe to deliver the ballistics for which they're made in the first place. A chopped 7RM would tread on 280Rem territory pretty quickly I think.
To get what you're looking for with what you have, load some bonded/partitioned/mono-metal bullets in 165 grain weight and go to it.
Also, copy 1899 on the recommendation to use El Cheap-O pills for practice, and the good stuff for load development, zero confirmation and game only. I buy literally whatever generic C&C or FMJ bullets for volume practice; the load hits nowhere near the same place as hunting loads, but that's corrected in about 3 shots when switching back and forth.