Really solid work! Lots of thematic development and variation, and the sound design is on point. The only thing I'd suggest is punching up the low-end a bit in mastering, but otherwise great!
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Ultimately, mastering = EQ + loudness control (and EQ is just loudness control in the frequency domain). You did well! In terms of technique - a multi-band compressor with a bit of extra bass crush and makeup gain in the low frequency spectrum is all I'd really add to the tracks you did, plus a fine-tuning EQ pass to make sure there's no problem frequencies that would mask loudness readings. Then run a limiter over the whole master bus until you get the LUFS readings you're after. (For sound cloud, about 14.5 - 15db LUFS is about right.)