My first rifle was a 300WM, I wouldn't recommend it, but it worked OK for me. I never developed a flinch, but I would say the recoil took a few trips to get used to.
Grains is the weight of the bullet. There are 7000 grains in one pound. Typically the lighter the bullet, the faster it's going, so you don't really reduce recoil much with lighter bullets, unless you get into loading your own and make some reduced loads (Which was one of the things I did to get more trigger time with the 300WM).
Partitions are bullets that have a soft point in the front, for expansion, and a sealed lead core in the back, that retains weight but stay the same shape. Best of both worlds theoretically.
Boat Tails are bullets that have a kind of boat tailed profile to the bottom of the bullet. This is done to improve aerodynamics. If these have a soft point they will be good for hunting. If you ever see Match bullets, they are typically Boat Tail Hollow Points, these are not suitable for hunting large game, as the hollowpoint is an aerdynamic feature, not meant for expansion.
If I were going to buy a lighter caliber rifle that would still take large game, I'd personally probably get a 7mm-08. But if you can try some guns, that'd be the best way to start, see what a magnum feels like compared to a non-magnum, I saw a 300WM for sale that had less than a box of ammo through it. It's not for everyone, and 300WM isn't even that bad compared to some of the beasts out there