I have been using my hdplex h5 for 9 years now (!!!), went through many builds. My current desktop is a 13900T with an RTX 3060, all fanless; 99% of the time, it's been great. I realize after such a long time that on occasion (1% of the time), I want all the compute power that this build has and I can't extract it from this build due to thermal throttling. The motherboard is a bit old (h670) and I need thunderbolt.
So now I'm planning to use the opportunity that the motherboard upgrade brings to "semi-silent" or "silent almost all of the time", with a Jonsbo Z20 + NH-P1, keeping the CPU but using an ITX z790 mobo I bought in ebay. Planning to inlcude an undervolted gtx 5060Ti dual fan with "0dba support" (no fans below 50 degrees), and two noctuas, intake at the bottom of the case and exhaust at the rear. Fanless PSU (seasonic, 500w). My desktop OS is linux. Home desktop machine. A little steam gaming. A lot of photo editing (#darktable)
Anybody in the community with experience in nh-p1 / semi-silent builds that can share real-world experience, will it able to handle 106W (13900T peak) keeping the CPU below throttling?

Not sure why you're so tied to a (mostly) fanless build with the upgrade. In my experience, ITX cases need airflow, because most cases aren't designed with fanless operation in mind (unlike your h5).

The NH-P1 does fine with 65W CPUs, but I think you will be disappointed with 105W+ options, especially if you have desires to push your parts a little. Additionally, it performs best with some active airflow, however slight.

I understand the want for quiet operation, so as someone who tried a 65W cooler on a 105W CPU, just get a better cooler. I'm running the Phantom Spirit 120 SE with Noctua fans on an undervolted 5700X3D, got a triple fan 3060ti TUF GPU (with fanstop), a 650W Cooler Master PSU (with fanstop), dual top exhaust Noctua fans, and I'm running custom fan curves with CoolerControl (I'm on Linux, too).

The whole thing sits about a foot from me, and I barely hear it with bare ears. With my non-ANC headphones, I don't hear it at all, unless I'm stress testing it.

Personally, with how hardware prices are going, I wouldn't trade the peace of mind of active cooling for the diminishing returns of passive silence, especially if I planned to actually push the parts at all. Keep those parts cool and working; don't make them rely on their built-in thermal throttling!

Hey, thanks for the comment! Well for me this is my first step in the active cooling direction in a decade, I will include two fans, and the case is fully meshed. I went with the all-mesh version, no glass. The GPU is dual fan.
I am expecting it to handle heat better than the H5 that forced me to power limit both CPU and GPU (both with passive cooling) and couldn't be pushed at all, and wouldn't accept fans. My hope for this new build is that it will transition from 0dba to some reasonable noise level with both fans at full speed when pushed to the limit and still sustain high load with no short freezes.

Why 0dba most of the time? Music fan here with a reasonable HiRes audio collection and nice speakers and the home office is my music cave. I have always hated computer noise while doing day to day compute, so many years back I got on a mission to eliminate all noise and succeeded at that with the H5. A decade ago there was nothing remotely comparable to alder-lake efficiency (and more recent iterations), the concept of fan-off most of the time didnt exist, I think. I believe this CPU with the Noctua nhP1 / z20, undervolted efficient GPU and on-demand fans will handle day to day compute and light pushing silently and better than with the H5, and will allow squeezing the CPU/GPU when necessary, at the cost of some noise.