Thank you for you kind comments. I'm sure ASUS enjoy the free technical support (if you can call my comments that !).
(1) The problem is that failure of component and massive temperature rises go hand-in-hand and being polite the causation is often assumed as the cause by untrained people (not trained in electronics/engineering). Thus with an added anxiety of a failed pc component from past or recent present and the correct observation just prior of elevated temperature, people have easily become focused on temperature believing this was the cause (lower is better mantra).
The truth is within specification is fine. Even when a CPU or GPU go beyond safe temperature they throttle before damage, or even shut down the system. Even this isn't dangerous or bad, it is protected and safe, when you resolve the cause, the system goes back to running normally.
If your system has bad component, it will fail, hopefully before warranty and you get it replaced. (and if you have really good consumer laws then even outside of warranty isn't safe for manufacture of a defective component)
As long as the system is running within the design thermal specifications it will last exactly the same time regardless of where it exists within the acceptable thermal (upper or lower) limits. As long as it within the these thermal limits, it will last the designed time or longer, albeit component failure or excessive o/c or user caused failure or (in the past by fair few companies) bad design.
(2) I use Arctic Silver 5 (since it came in '99 varies versions) but Grizzly Kryonaut is better, enough so, that I am considering switching (seriously). I use coolermaster for AIO's and have for about seven years. I have this AIO
https://www.coolermaster.com/catalog/coolers/cpu-liquid-coolers/masterliquid-ml360r-rgb/ however if money is no object this one has fan (Noctua) built in, which cools the VRM
https://www.asus.com/us/Cooling/ROG-RYUJIN-360/(3) They are ugly as but they also work (I've used them in the past) nearly as good (2), less moving parts (points of failure). However that said, my minister for war & finances, has decided AIO's are in for the last eight years and ugly and clutter around the motherboard is out. I know where I like to sleep and there is no way I'm going to disagree with her or argue my points made before and win.
(4) Till it's broken through failure, you can't normally RMA and not have it returned or at least not here in Aussie world and we have some fairly good consumer laws.
I really don't think what my system get matters, we have some shared components but different cases and climates. Anyway heading into summer and the typical temps are rising now with warmer days, I see at the moment in gaming, (via Aida64) between 60 to 72 PCH Diode. M2 runs 32 to 40. The max for GPU 64 and CPU 62 (or lower). Both the CPU & GPU are on AIO's there is less temperature being dissipated near the motherboard components and I have fair airflow through the case but can make that extreme if needed (never needed).
You system seems fine to me for temperature given. I see nothing that worries me.
Hope you find this of help.
PS: There is new bios out early next month which if rumors are anything to go off sounds very positive improvements.