For all following suggestions you need a 1.8v power supply (buck converter). First, you could disable internal pullups on the stm and remove all pullups on PCB2 for the signals from the battery ic. Because the signals are probably all opendrain you just need to pullup the signals on pcb3 to 1.8v; either the stm or the battery ic can pull the lines low to "activate" them. The stm should recognize 1.8v as high ...

Second, you could add a txs0104 or two txs0102 (probably cheaper, you can get those from aliexpress) for level shifting. They have internal pullups, so you can remove the other pullup resistors as well.

Julius Lehmann