After extensive research into technical WAD stuff, I made a number of fixes to this .wad (in SLADE) to get it to work with DOS Boom:

1. Exported the textures in the .wad for later re-importing
2. Deleted all the non-map entries from 0-12 (the markers and textures)
3. Re-imported the textures and converted them to paletted Doom Gfx format (disable transparency)
4. Recreated the TEXTURE1 lump, importing from doom2.wad base resource
5. Added FF_START, F_END, PP_START, and PP_END markers at the end
6. For the OMINOUS and DIRT textures, since they're used as flats which are handled differently: resize them to 64x64, import them, convert to paletted Doom Flat format (disable transparency), and put the lumps in between FF_START and F_END

This was...a lot to figure out through trial and error and Doom Wiki research, but I guess now The More You Know™

thank you!
i will come back to this if i ever try custom textures again... .. . . . i wish it was simpler ahahah