h solar

I would think you would be able to restore the correct firmware variant without significant issue. The EEPROM would not be able to be read, so the firmware would immediately throw up a warning, I think, but the serial code should still work. Best not to have anything connected other than the battery.
Firmware versions 12.21 and later are "universal", i.e. they work on either chipset.

These files are now quite old, so I wonder what you need these for.

This is display/comms (U2) factory firmware version 12.06 for the Axpert Max non-Twin models with a removable display. Do not use with any other model or version. This version cannot replace 112.xx firmware.
Use at your own risk. The hex file is dated 04/12/2020.
Firmware upload instructions for models with a removable display.
This firmware does not process the QDOP command, so even with suitably recent main firmware, it will not support SoC% for settings 12,13, and 29, when connected to a suitable battery's BMS.
MAX3k6-7k2-Remote Panel_Reflash_MCU(12.06).rar

Some more schematics here. These are from Easun SMW 8kW which is, I believe, identical to Axpert Max 8kW. In this case the control board did have part references printed so most of those in the schematic should be correct. It's a multilayer board but only power planes are on the inside layers so it's not that hard to trace. The main difference, compared to Anern, is that this board uses multiplexers for 8 of the ADC's inputs, increasing total number of available analog inputs to 24.
I also made a schematic of the SCR board and some fragments of the main board.
I'm not going to make schematics of any power supplies as these are already pretty obvious. Just note that the topology in this case is different from Anern's. There's no intermediate SPS_IN (150V+) branch. The main power supply and bus softstart are located on the main board and powered directly by the battery voltage through a diode. The grid/AC power supply and solar power supply are on a separate board, both of these output around 63-66V and also provide power to the main power supply through diodes. Unlike Anern's, this topology does not need a relay in the softstart circuit.
I made a note of some critical parts (MOSFET type, shunt resistor value and how they deal with voltage spikes):
main SPS: JCS86N25WT, 0.15R, RC snubber 2x47R (parallel), 2.2nF
softstart: SIF13N50C, 0.62R, RC snubber 2x22R (parallel), 220pF
grid SPS: SVF9N90F, 0.47R, RCD clamp 15k/5W, 10nF
solar SPS: SVF9N90F, 0.47R, RCD clamp 15k/5W, 10nF
Original KiCad files and PDF schematics are available on my GitHub: https://github.com/solar-pwr/axpert/
There will be errors as usual, so every feedback is appreciated.