Welcome to guns and the forum.
Some members will be hard on you - I'd suggest dialing it down just a little on the verbosity.
Gun folks don't really talk or write like your original post. From the sounds of your post, it sounds like an advertisement for HK .45 ACP and the .300 blackout! Neither of these high priced weapons/caliber will do anything that weapons/ammo a 1/2 the cost will do for a civilian for self defense purposes. I'd suggest re-thinking this. Clearly you're new to guns. The 9x19mm will be 1/2 the price in ammo, allowing you to buy more ammo and train more. Same is true with the .300 blackout. If you can find it, it's going to be very expensive. Pick a more common caliber. Wars are fought with the very effective 5.56 and 7.62x39. I'd suggest an affordable SKS or AK47 and a pile of ammo. Then get to the range for training.
Your name is "Safetyfirst." Safety comes with training and trigger time.
A few thoughts. The 12 gauge is an incredible and versatile weapon. I'm a well-trained Army combat veteran, and in my personal arsenal I have many legal weapons to choose from. But the 12 gauge is definitely in my starting lineup. (Note: I do NOT have a 300 blackout. I see absolutely no purpose for this niche caliber in my lineup. It does zero that my .308 and .300 win mag and 5.56 and 7.62x39 won't accomplish, yet it is incredibly expensive and rare. You need versatility in your ammo selection and weaponry). The ancient AK47 with the 7.62x39 is hard to beat for home defense. Weapon, ammo, and mags and parts are widely available and affordable. That is also in my starting lineup.
There definitely is validity to using a long gun for home defense, and learning to use one effectively. While it is unlikely to ever need to shoot 300 yards, that is a skill you should have. I personally believe we are due for SHTF civil unrest on a large scale (for a variety of reasons, none important here), and we all can remember when the shop keepers in the LA Riots in the early 90's had to defend their property from marauders from rooftops with long guns. So you should be able to employ a long gun if it's called to duty. While unlikely, that's all I'll say about that.
You'll find that most people here will tell you that the equipment (high end pricey gear) is not as important as practice. If you have $10 you should spend the majority on ammo and practice and shooting schools. An inexpensive handgun/rifle and lots of trigger time is better than the priciest weaponry and no training.
Get to the range, take some classes/shooting schools, etc.