I think this track has a lot of promise! It sounds like you have basically everything you need. If your song was a cheeseburger, you have the bun, you have the patties, you have the lettuce and the tomatoes, you have the ketchup, and you even have a sprinkling of salt and pepper in there for extra flavor.

Unfortunately your lettuce is a whole leaf, you have a single cherry tomato, your beef patties are different sizes, and your bread is too thin. Take some time to look at how the whole song should sound in relation to each other, and go from there.

Beyond the mixing, you should take some time and pick something that can unify the song together, and can give it something memorable the listener can hold onto. Look for some reference tracks that might help you if you're having trouble (Jay Fay, Dillon Francis, and Foxsky are some good Moombah artists that you could look to).

Overcomplicating things is a super common problem for producers - especially in the age of DAWs, where you can just keep on adding more effects or layers with little to no impact on your performance (for the most part). It's why you will see people encouraging arbitrary restrictions - if you explicitly restrict how much you're able to do, you won't be able to do too much! 

If you can't get past the introduction phase, it could also be that you're just unsure of where to go - so use a reference! Pick two or three track that have the vibe you're going for and use that as a guideline. You don't have to copy it exactly, obviously, but there's no reason to work inside of a vacuum when you feel stuck.

Everything I've grown to understand about mastering is deceptively simple, it's just a bit tedious and repetitive.

A good master is more about the songs you reference than it is the processing chain you use to master the project. Take two or three songs that sonically hit your ears in a way that you want the song you're mastering to sound, and bring them all into a separate project with your premaster bounce. With your mastering chain running on your premaster instead of your master bus, you can now A/B between your own track and your references without coloring the references at all with your processing chain.

From there, it's all a matter of EQ, stereo imaging, compression, and clipping/limiting to bring the song up to commercial loudness.

EQ: If you're really lucky and have a stunning mixdown, you really only need a steep (4pole or -48 dB slope) low cut at 20-30 Hz and high cut at 20 kHz to remove any signal that resides outside the range of human hearing (if any is left inside your track - you can do this at the end of your premaster). Beyond that, this can help push your song just a little bit closer to the frequency curve of your reference tracks - though if you're trying to make changes more than +/- 2 dB, you should probably go back and fix it in the mix. Mostly try to use your ears and figure out if their different frequency ranges are louder or softer than yours, and EQ to compensate. You can use a spectral analyzer to get an even better idea after you've honed in on where you might need to go.

Stereo Imaging: this is gonna be very light touch, if touched at all. Ideally, you have all your stereo imaging sorted out by the time you're getting bouncing a premaster. All you're doing is trying to help spread out a little bit of the higher frequencies, and make sure you're not having too much phase cancellation from the stereo image when you collapse to mono.

Compression, Clipping, Limiting, Maximizing, et al.: These are all working with the same goal in mind - Make Things Loud. That being said, you want to be gentle here too, and really pay attention to your references. Each one of the things listed has a different effect, and you should take your time reading up on what they do and experiment with them in different orders. This is also an area where stacking multiple is often better than relying on just one - a pair of limiters with different attack and release settings (one fairly fast to focus on the transients and really grab those peaks and valleys, one slow and steady to work on the rest of the track as a whole) each getting pushed by 4 dB will give you about 8 dB of loudness while essentially imparting the fingerprint of a 4 dB boost, because you're spreading the workload between the two.

If you're A/Bing the entire time you're going through each of these steps, by the time you're done you should be able to jump between the references and your now-mastered track without any apparent change in the loudness or frequency profile. From there, you start testing out your master (and your reference tracks) on as many different speakers as you can get your hands on, and use a notebook to keep track of everything that you notice is different. Listen on your laptop speakers, on your headphones, on your studio monitors, on your earbuds, on your phone speaker, in your car, in a club or at a festival (if you're actually lucky enough to get that opportunity), or wherever else you have the opportunity. Listen to it at low volume, at medium volume, and at high volume. Go back to your DAW, make some adjustments, then rinse and repeat until you don't hear any differences at all. Take a week off of the song, then come back and give it another listen through all the same channels, hopefully one last time to verify that it's all good.

Congrats! You're done mastering this song, and can now do the entire process over again with your next song.

Instead of thinking about how many sounds are too many sounds, why not try shifting it the other way around?

Try asking yourself "how few sounds can I get away with using while making sure this still sounds like a completed song?" Sometimes you'll need a wide variety of different sounds, sometimes you'll need a lot of layered sounds, and sometimes you'll be able to just have a single sound source going through a bit of processing (if any) - and they're all completely fine.

Don't look at it like you have some quota of sonic information you need to fill. Look at it like you're trying to avoid adding anything new or complicated. In fact, any time during the song creation process if you can simplify something without compromising its integrity I have a sneaking suspicion you will find the simpler melody and the simpler sound source and the simpler processing will be more appealing to you and to your listeners.

Just don't conflate simple with basic or easy.

Good old filter and volume automations can never go wrong.

Lately I've been experimenting with delay tails over sections that have the drums pulled out as a way to clearly announce the introduction of a new section while maintaining a less rigid boundary between my instrumentation.

I would definitely go seek professional medical advice on this, as there are really only two reasons your hearing would be deteriorating at the rate you seem to be facing

1. Some degenerative disease has attacked your hearing, and you should seek medical attention for that immediately.

2. You are listening to your music WAY too loud or exposing yourself to extremely loud volumes with absolutely no ear protection.

In both cases, you need to really pause and figure out what is going on, because it's going to be very hard to write music if you can't hear music. Not impossible, of course, but you should try to preserve as much hearing as you have left.

E or F# would work - B is the perfect fifth of E, and F# is the perfect fifth of B.