UPDATE:
Guys, I got ESPHome OTA updated onto my Shelly Plus Plug US, FW 1.7.5 without any need for Tasmota.
I used GPT-6 Sol to inspect the stock FW mappings using Ghidra, then it seemingly injected the ESPHome app into Shelly’s partition mapping app0, and then ESPHome booted up just fine using Shelly’s custom ESP-IDF 5.4.1-s4.
After that, ESPHome’s OTA bootloader update works and I now have a fully unlocked Shelly running ESP-IDF 5.5.5.
I am still figuring out mappings for the hardware but we are getting there, and I will make a post on how to do it once I get it verified on my other Plug USes.