Credit to @Heimdal for helping me figure this out!
FYI for everyone using these instructions. if you have had issues where your ESC with rental FW will not connect to STLink, follow these directions below:
Step 1: orient the ESC so that the connectors are on the bottom left corner and giant capacitor is on the top right. remove potting on the lower side of the processor and the components near that area (see photo for reference). the capacitor circled in red is the only important one to be reachable.
https://imgur.com/a/UU4IT9d
Step 2: solder GND, CLK, SWIO, and power (5V on white connector or 3.3V on cap near CPU, whichever you prefer) and plug ESC into STLink into PC as if you are going to flash.
Step 3: use metal tweezers or anything metal to short the two ends of the circled capacitor together, this will force the system into reset as long as that's held.
Step 4: while shorting the capacitor, click the connect button in ST Utility (the plug with the lightning bolt), if a read protection error shows up press Cntrl + B and disable readout protection as usual, then press the connect button again.
Step 5: nothing will apparently happen in ST Utility after pressing the connect button. remove your tweezers from the capacitor to allow the processor to come out of reset, ST Utility will then connect to the system and you can flash and do everything as usual!
enjoy! this also works with ESx, i will post instructions and a photo there later.