Is it possible to load a basis set from an external file rather than pasting it into a $basis section and making the input file rather cumbersome?
I’m not sure, but what you can do is to define either of 2 custom basis sets, named basis1 and basis2, by putting basis1.bas and/or basis2.bas into $QCAUX/basis (i.e., into the place where basis-set info is stored for your local copy of Q-Chem). Then you can access these by setting, e.g., BASIS=BASIS1 in the $rem section. This feature is limited to two custom basis sets.