Ok, another workaround would be:

map(0x0000, 0x7fff).mirror(0x8000).r(FUNC(arb_state::cartridge _r));
change to:
map(0x2000, 0x7fff).mirror(0x8000).r(FUNC(arb_state::cartridge _r));

and in cartridge_r do:
offset+=0x2000;