Why run it at 5Ghz with all threads enabled when it is simply not needed right now?
I only change my PC's every 4-5yrs, I don't want to rip out the motherboard & CPU in a few years time to upgrade, and buying a 9900K now means I won't have to do that because I know I have plenty of grunt left in reserve ready to be accessed when needed. It's just a waste of power running something at 100% capacity when not needed. As it is, the ONLY thing I'll need to upgrade during the lifetime of this PC, barring hardware failures, is my RTX 2080.
You are absolutely right in your decision to keep the CPU at more conservative and silicon-friendly settings.
Once you see CPU bottleneck, you can return back to the higher frequencies. But meanwhile, your CPU will be cool and safe, and this will elongate its life