Welcome to the rifle phase. First off you are starting with a very good set up. Howa's are sub MOA rifles. All rifles have there own little quirks if you will.
1 in 12 is a comon twist Should be able to run well into the 60 to 65 gn bullet weight with it. Picking a powder and primer and bullet is the biggy.
What i do when i first get a new rifle is google your model and see what others are shooting and reporting on it. KInd of gives you a general starting point. This is not a plug,but go to reloaders nest.com. There are a multitude of loads there and you might find some one that is shooting your same rifle.
Take load data from that sight with agrain of salt
. 1 inch groups at 100 yards should be a cake walk for that rifle. Since the farthest you mention is 300 yards i would think you should still be pulling less than 3 inch groups with ease also. Your brass pick at 100 yards will make next to no difference,that will start to come into play at a distance. Hornady or Nosler or Serria,,Never seen a dog care which one he gets hit with. Shoot the one your rifle likes and go with that. Myself, I shoot 60 gn Hornady's. With a 223-powders are all over the place, H335,Varget,Tac,RL-15, ect ect. Read Dan Newberrys load development and have fun.