I think you may just be confused if your head is spinning. The basics are very simple to grasp. You have two elements that will fight for priority. First is cost, and the second is performance. Antennas come in various different designs - mainly separated by gain, not really quality. You can get commercial grade antennas that are made stronger, and maybe even using better materials. A typical amateur antenna might cost between 25 and 100 dollars/pounds/euros a commercial one could be 150-500! The commercial 3dB antenna will perform the same as a cheap imported ham antenna also rated as a 3dB design - but the commercial one could be still working 20 years later.
Higher gain means your signal gets further, and you can hear longer distance folk. Usually it’s done by concentrating the beam by not pointing signals up at a high angle where they do little, but bringing the radiation pattern towards the horizontal, away from the vertical. It doesn’t ‘create’ any extra power, just sends it in a more useful direction.
The really important thing is where the antenna is. The top of a mountain is great, the river at the bottom of a valley is bad. If the antenna is up in the clear, that’s best. Topography is really vital. If you live somewhere low, surrounded by high things, your range will be bad - even with very big, gainy antennas. My office has tall buildings on one side of it, so I get great coverage out to sea (useful for my marine band equipment) but inland my antennas are lower than the roof of the building next door. There is a repeater 8 miles away - totally absent!
If you are on the top of a hill, then an antenna with no gain at all will work surprisingly well. A really good antenna will reliably bring in longer distances. The thing to remember is that if there is no signal - even the highest gain antenna is pointless. They might take a weak signal and bring it up to something workable, but they are not magic.
Cable and connectors can waste your signal, so if you lose 3dB down the cable, with a 3dB antenna, it’s like having a 0dB antenna and no loss in the cable. Ideally you want the most signal you can get, so very low loss cable and a big gain antenna is the best - AND - the most expensive. In most cases a moderate gain antenna with medium loss cable like the 400 you mention will be the most affordable solution. If you have a budget, then spend it wisely. If the topography is bad - then it’s going to be expensive. If you live on a hill - how much range do you need/can you afford.
Worst case = low gain antenna, poor performing cable with lots of loss and an antenna location that is compromised by shielding and height. Best case = high gain antenna, virtually no cable loss with the antenna above all local obstacles on the top of the highest ground in the area.
You won’t find getting that ultimate scenario easy at all.
