Ahoy, LeVerrier...
Like propbeanie, I can attest to the fact, that ReShade, does work with being used with a modded SH4.
I use a ReShade preset with Fifi's Dark Waters Op. Monsun version...
Haven't used it with FotRS-UEN v1.39, because.. well, the preset is set up for Op. Monsun: Dark Waters & not FotRS-UEN...
BUT... the main point is, it does work & works well, with SH4.
Can't find the preset I used right off, but know, it is NOT that difficult to set up... @ least, it wasn't on My end of things...
Hopefully, this info helps...
M. M.