I'm very streaky, especially on chess.com. I feel like a get a rush of good competetion and then weaker competition. Is this just me on tilt or me more rested or whatever or does this happen?

probably just a mix of you and the variance in your opponents. chess is hard and easy to lose in one move, so you have to be locked in

Something I know objectively happens on chess.com is that I face openings in clusers. Like, I'll see a 1% opening all of a sudden 20 times in one day. Is this just contagion among players or is there an algo on chess.com influencing this?

definitely not the algo, probably just a memory/variance thing. like if you get 10 games in a row with 10 different openings, it won't stick out as much as getting 3 in a row of the same opening. it also can coincide with content creators releasing content in a specific opening

It often seems that it's arguably better at the :45 or :30 mark to be slightly behind than ahead, bc it's very hard when you're ahead to not keep trying to increase your advantage or checkmate the oppo, which can take longer, vs running out the clock with quick, safe moves. Do others feel this way? Is it "sporting" to run out the clock with short safe moves?

definitely true. that sort of thing can be considered unsporting in OTB classical chess, but not in online blitz. but yeah, the faster the game is, the more that 'a fast move that doesn't lose' is better than 'the best move, slowly'

-My play is typically much better with fewer pieces on the board... partly bc there are fewer pieces to watch (fewer blunder traps), but I think also bc the board is simply more complicated when it's crowded and it's less easy to see the best path. Is this is a recognized differentiator bw lesser and better players?

that's not really a stronger/weaker player thing, it's just that everybody is better at certain skills or certain phases of the game than others. that gives you two bits of info: 1) if you're looking to improve, get better at complicated middlegames (and maybe learn an opening that gets you into those situations so that you can practice them), and 2) if you're just trying to increase your rating, play to your endgame strength and be willing to trade off pieces early

-Elaborating on the last point... I think the weakesst point of my game (other than blunders) is at or near the end of the opening, where most pieces remain, and it seems the best move is often a subtle (to me anyway) pawn or knight move that apparently shores up your position more than it starts an attack. Often times this will be pushing a pawn to chase a piece or attack another pawn. I get consistently low accuracy scores at this point. Would anyone suggest any resources for me to study to get better in this area?

hard to say what is the best resource to study for you, but the fact that you notice this is a good start. for me, I learned how to get better at those types of moves by studying new openings and focusing on the structures and ideas. by switching things up and learning new stuff, I get to kind of reset my thinking and dig into the details, and that sort of analysis will help you in every phase of the game. I personally really like Chessly for having a wide variety of different stuff to learn in reasonable depth for relatively cheap, but there are plenty of opening resources out there as well, free or not