This reminds me an old issue I had with E-Cores, however with more frequent peaks every few seconds. Other users faced similar issue as well at that time as we also had a couple of topics about that. What you see is a GPU drop because most likely there´s a drop on CPU side indeed, which is the real cause of the spikes. I have a 12900ks and two days ago there was a new BIOS update including intel microcode update for 13th/14th gen CPUs, together with new Intel MEI drivers. You could start with that and check if you have the very latest updates as well.
The old issue I had was dealing with P-Cores and E-Cores. I saw you disabled your E-Cores but rather than disabling them, which can reduce your overall multithreading CPU performance, you can try to manually set the cores behaviour instead. Windows scheduler tends to move workload from P-Cores to E-Cores based on activity, which is not always the best approach, at least in our simulator case. You may want to manually force long threads on P-Cores and short threads on E-Cores instead. I explained how to do it here:
Last resource I can imagine is a periodic autosave feature, triggered every minute. I also saw you removed third party apps but double check that nothing related to game was left that could be performing such autosave on the background (e.g: On Air, FSUIPC, etc). In theory apps not related to game would not be causing this unless you are saving tons of data to disks, so I would ignore them. But most third party apps directly talking to game use SimConnect interface. In developer console you can see that, even under normal conditions, SimConnect can still have a quite frequent polling (multiple times per frame indeed).
I know this kind of issues are really annoying as they ruin the whole experience. I hope you can solve it and good luck!
Cheers