Curious as to what model SBC this unit is using
8 bit guys teardown says the internals are 'an ARM computer similar to a Raspberry Pi'
Per a forum post:
THEC64 Mainboard is based on this development board
ROCK64 is a credit card size 4K60P HDR Media Board Computer powered by *Rockchip RK3328 Quad-Core ARM Cortex A53 64-Bit Processor [...]
Further details are in that post, and the development board it is referring to appears to be this one though the board from the C64 Maxi looks like a unique design, adding a heatsink and what appear to be solder pads for 9-pin joystick ports (presumably repurposing the GPIO) and omitting an ethernet connector (though it looks like that's what the other solder pad is for?).
That should give you some decent information on the chipset though.
EDIT: here's a direct reproduction of the high-resolution image from that forum thread, in case somebody who is better at this stuff wants to verify my guesses about solder pads: