Pity about the Linux GPU support, but it would also depend if the apps you use took advantage of Nvidia cores over AMD cores anyway.
Thermal paste varies greatly between manufacturers on the materials used, and the nano granularity of the active ingredients, but obviously all that comes at a cost. Better paste reduces temps further by a margin, perhaps one or two degrees (assuming you have a compatible cooler), this also ties into number 2
As already advised unfortunately PCS don't currently offer premium air coolers, it wouldn't be suitable for your configuration and you'd get thermal throttling and therefor, not the full capabilities of the CPU as a result as well as significantly increased noise, this is even more important in that specific case as it's audibly damped, it has substantially reduced airflow and isn't suitable for air cooling anyway. Tying into number 1, the previously suggested cooler comes pre applied with high quality paste already which is why that wasn't suggested.
The 7900 is a very efficient processor, it's exactly the same silicon as the 7900X, just optimised and power limited, but you don't lose much performance as a result (maybe 7% under all core workloads), but even then, you can simply apply an auto PBO overclock in the BIOS (simply toggling on one option) and you'd have exactly the same performance as the 7900X. It's a great processor.
There's premium, and budget, you definitely get what you pay for, but assuming premium, they rarely break these days, 10 years ago when they were new, leaks were fairly common, leaks just don't happen anymore and haven't for years. Pump failures do sometimes happen, but again it's very rare. You have to be careful that you put what you read online in context with time frame, and the specific AIO it's talking about. You also need to be sure you're not confusing an AIO with a watercooled build as they're not even remotely similar. My last system had an H100i in it and was on 24/7 for 10 years. My new one has an H150i, but for noise the larger 140mm fans on the H115i are a better option as they need lower rpm for equivalent airflow
That couldn't have been a Corsair then, must have been some cheap one, or I think you may be getting confused with a custom loop which is an entirely different thing. Corsair coolers don't have liquid in them, they have non-conductive coolant, so even if it was to leak, it wouldn't cause any damage. There's also very little coolant in there anyway, enough to make a tiny puddle, but certainly not any issue.
Then you'd need a much lower processor if you wanted a PCS build, anything above really a 7700 just can't be cooled by air, even a premium cooler. Or you could just fit a basic cooler and then source your own premium cooler like an NH-D15 which if you want quiet running is really the only choice IMHO
Urm, because you use an AIO as intended, air cooling is rarely used these days as it's just not effective for anything but entry level processors. Air coolers are also far more noisy than an AIO, unless you're talking something extremely high end like a noctua, but those cost pretty much the same as an AIO anyway.