Hi 3DGuru. As far as the fans are concerned, I think you just have try some different software and see what works best. I have a Corsair cooler and their software works great - for speed and fancy RGB effects. Mind you, the RGB excitement wears off pretty quickly to be honest lol. I like Armoury Crate, but it only sees fans that are connected to the motherboard on my setup, which are just two small PCH cooling fans that I added and they don't have RGB anyway. I mostly like Armoury Crate because I can see the drivers at a glance (and to control those 2 small motherboard fans).
Re: the cooling I agree with Silent Scone, that's a good idea. Again, I don't use AI Tweaker and I've never experimented with it to regulate the CPU temperature, but it sounds like a very good way to do it.
I don't know why HWInfo and your Ryujin cooler would show different temperatures - I would have thought they would both pick up the same sensor, so I can imagine that your cooler is using it's own built in sensor then, but I don't know, that's just a guess. Usually, it's the 'CPU Package' temperature that is used by most software.
If I run Cinebench, then I get the same temperatures in Corsair iCue, HWInfo and OCCT software. They all give steady readings of about 85C at the start (reached in a few seconds) and if I leave the test running they gradually climb close to 90C. I think that I would trust HWInfo, as that will certainly be reading the CPU temperature directly from the CPU package itself. You can get rapid temperature spikes if the load on the CPU is varying. It takes a while for equilibrium to be reached under steady conditions because the heat has to be transferred from the die to the CPU lid, then the cooler and the water circuit. The die temperature can change very rapidly within all of that 'wrapping'. The cooler efficiency will make a big difference overall and so will ambient temperature.
It sounds to me like everything is going well so far. While we are taking about temperature, if you want to limit your GPU temperature (or the power draw) then I'd say use MSI Afterburner, it works like a dream. I limit my GPU to 65C and I get great performance, even during 4K gaming. Be careful when you download it, there is a fake Afterburner site.
This is the real one:
https://www.msi.com/Landing/afterburner/graphics-cards
Z690 Hero, BIOS 2305, ME Firmware 16.1.27.2176, 7000X Case, RM1000x PSU, i9 12900K, ASUS TUF OC 3090TI, 2 x 16GB Corsair RAM @ 5200MHz, Windows 11 Pro 22H2, Corsair H150i Elite AIO, 4x Corsair RGB fans, 3x M.2 NVME drives, 2x SATA SSDs, 2x SATA HDs.