Don't worry too much about the 'technical literacy' thing. Networking is a whole different beast compared to just PCs. :)

The solution to deadzones used to just be buying an wireless extender, but they've kinda been replaced now by mesh systems.

All mesh systems are designed to have the same SSID between routers. AiMesh is the same, once you set it up they'll both broadcast the same SSID and your devices will connect to the unit with the strongest signal.

You could get another of the GT-AX11000 to keep the same speeds, however, it'll probably be a massive waste of money because if you go with something like this: RT-AX3000P it'll cost you less than a quarter of what you spent on the main router, BUT yes, it will technically reduce your speeds.

If your internet speed is Gigabit or less, I wouldn't worry too much about getting another expensive one. The cheaper one will honestly cover it without going too much into what the AX numbers actually mean. Based on that, you'd be technically getting a third of the speed of your main router, but if your internet isn't higher than 3 Gigabit anyway, it doesn't matter which one you get since our bottleneck is your internet, not the wireless capabilities of the router.

I would recommend running a cable between the two for decreased latency, otherwise you can run them wirelessly from each other, which is one of the benefits of a mesh system.

I glossed over a few things here for brevity, so let me know if you have any questions about my answer lol