Nothing wrong with building your own. I've built two off of an origin and have no regrets.
I think you're way overkill on the cartridge but if you're dead set on it, there's no reason, not even factory ammo availability, to go with a 300wm over a 300prc. One of my origins is in 300prc with a 22" proof barrel. Getting 3100fps with TTSXs and 2860 with 215 Bergers. That's only with 30 shots on it so it will speed up a bit.
With that said, if you really want to practice long range, that cartridge isn't for you. For 600 and in there's nothing wrong with your .308. But I get where you're at. I've been there too.
Think about skipping the magnum 30cal and purchasing a very high quality suppressor. You want to prioritize length and weight with suppression performance. I'd recommend the AB Raptor 8 or a Silencerco Scythe.
With the extra money you have left over, buy reloading equipment and start teaching yourself. It's a very rewarding hobby and you end up with ammo that's MUCH higher quality and tailored to your specific goals.
Or hell, do all of the above and build a 6.5prc.
Here's some origin pronz for you to help you make a bad decision.