So I've still been trying to play more aggressively, and the good news it that it's led to a string of victories against KGS 6k players that finally got me bumped back up to 5k for the first time in a good two years. (Whenever I put up a challenge, I was only getting responses from players equal to me or lower, so I didn't get to play many stronger players recently.) The mixed news is that I seem to be winning more because I'm doing a better job of spotting my opponents' mistakes than they've been of spotting mine -- in fact, I've been spotting some of my own serious mistakes during the game and hoping that my opponents miss them.
I had the sinking feeling that once I got challenges from 5k and especially 4k players that they'd be much more likely to spot my mistakes and take advantage of them. Still, I tried to play aggressively, and got a game where I felt I didn't play badly, and certainly had good chances to win -- until I missed at move 205 that L16 would become a fake eye, costing me that group in the top center.
Any other ideas on where I was playing too passively, or where I thought I was playing aggressively but really wasn't?
is neither here nor there. There's enough space that you should be expecting to live on the side/corner rather than run away, which is what you are implying by playing on the fourth line. B15 is a standard idea here that is worth looking up. Another modern-style idea is to attach at D11.
: Bravo! I don't know if it's the best move but it's great that you played it.
: This is the first "old Fedya" move I noticed. You are good enough to be able to read that a White cut here doesn't work at all. A hane at Q6 would make such a difference. (I just peeked with Leela Zero and it says that your move at R3 reduced Black's chance of winning from 73% to 29%. That's how bad it is. It's practically -1 point in gote.)
: What jumps out to me is the big moyo White is making at the bottom. I think that this is the first priority. Something like K4 seems a like a promising way to frustrate White. (I should note that Leela Zero thought your move was just as good, though.)
is the natural move, but the 
/ 
exchange just makes both sides more solid in a situation where black wants to remain light and flexible. Because of the presence of 
is almost necessary.
at b is supposed to do here. Prepare to invade at the 3-3? Otherwise it seems awfully low.
I checked with Deep Leela (Leela 11), exploring possibilities, because you can't rely on winrate estimates with low playouts. DL considers your invasion to be about 3% worse than the atari in the bottom left corner. By my estimation, 3% is around DL's margin of error, so I think we have to say that 
is a bit worse, losing about 4%. It prefers to play in the bottom left at "a" or "b". Locally it prefers the attack at "c" or "d". All four plays are playable, IMO.
. If 
could be at "a", the sacrifice might be better, I don't know. But it can't. White is too strong here and Black has become heavy. 
at a is also possible, and get prefered after a while (but with no significant winrate difference)