TDP is a measure of power usage when all the CPU cores are at 100 % usage. Most homeland sit idle almost all the time. Basically TDP is mostly meaningless.
You will find that your 9th gen Celeron idles pretty close to the same as a i3, i5 or i7 of the same generation.
The cheap upgrade would be an i5-9500 in your existing system with perhaps some more ram. Unplug the other system as it's really slow. Upgrade your drives.
The problem with a newer low power CPUs like a n5105 or N100 is a lack of pci-e lines and SATA. If you want multiple high speed m.2 and non multiplexed SATA, you need a more standard CPU. Consider a 12th or 13th gen i3.
I'm not against n5105 or other low power systems. I run several. They just are not the best for a NAS.