I want to hide a spell I get automatically from my sub class (specifically, Arms of Hadar for Aberrant Mind). Is there any way to just hide it when viewing my character sheet?

No, not without creating a homebrew copy of the subclass and switching to that instead of the official version. Why do you want to hide it? Maybe these will help:

Method 1: Swap As Needed

Method 2: Swap Everything

I'll check out the videos when I have some free time.

I was told that there was no way to retrain subclass spells on your character sheet, so I was hoping I could just hide the spell from my spell list and then add the retrained spell in with a quick homebrewed feat.

Otherwise, best I could come up with was to customize the display name to reflect that it was retrained (though it would still take up space).

Ahh. Better to make a homebrewed version of the subclass and use that. Then you can swap out any of the spells you wish at your leisure. Those vids should help.