What are you talking about? Those temperatures aren't dangerously high at all. 75C with a 1080 inside the Dan A4 is rather good, I have a 2080 that gets around 85 degress while gaming. Keeping in mind that this is one of the smallest cases on the market and doesn't exactly have any airflow at all.
And the users 3600X has a max temperature of 95 C, and if his CPU is doing 60C during idle and around 88C during load, it isn't very good, but it's not thermal throttling either. Especially considering he's using the Noctua NH-L9a, which isn't the best cooler.
And seeing as I bet the person hasn't manually tuned their CPU, I bet the user is running auto voltages, which are very high and therefore make the CPU run a lot hotter.
So no, those aren't "dangerously high" temperatures. I'd respectfully and kindly advise you to be a bit mindful of your words in the future, as you may not know how the other person might take them.
They might be so scared to even turn on their PC because of it (I used to be that person).
Regarding your own system, what exactly do you mean by "full continous load"? Are we talking about rendering? Gaming? What kind of game and at what resolution?... There are too many variables in a system to just say that your system has a specific temperature, since everyones system is unique and no two systems will perform exactly the same.
Why should the person put a 120mm fans under the case and on the sides?... If the person would undervolt their CPU, they don't need any extra fans and their temperatures will be better.
How do I know this? Cause this is what I did to my 3700X and 2080, which is using the Asetek 645LT and it never surpassed 90C during Aida64 Extreme for over 18 hours continous load. 90C is high yes, but not surpassing the 95C max temp. And keeping in mind that Aida64 Extreme is basically the absolute worst test you can put your CPU through. It literally pushes your CPU balls to the walls, the worst case scenario possible.
So having 89C for continous 18 hours of that stress test, it passed. And during full gaming load, my CPU is around 65C. Keeping in mind that at the end of the day, in this computer case, all cooling systems are extremely limited and high temps are unforunately not to our favour.
Lastly, I agree with you on the SF750 PSU. But the included cables, while they will get the job done, they are far from perfect. They are extremely long and will take up A LOT of space inside the A4 SFX. Getting custom cables helps immensely. I am very happy that I got custom ones, as they are just the correct lengths for this case and now my case barely has any cables at all.
Oh and by the way, please don't take this post as personal or like I'm attacking you. I'm just trying to shed a different perspective regarding your post, in order to hopefully give the user a bigger picture and understanding for his system, temperatures and ways to improve. Nothing else