When he’s being aggressive, it’s not that hard to land hits, but defensively I’m suddenly fighting it hard to land anything safe.
So this is actually a pretty common counterplay to Lucina, at least from my experience. She fares better when the opponent is rushing at her but when they start to play more passively and focus on whiff punishing she can start to struggle. What I would suggest is that you should focus on cornering your opponent rather than rushing at them. One flaw with bait-and-punish playstyles is that you have to give up some ground. Taking center stage is key because it limits where your opponent can move and evade.
he’s gotten a lot better at keeping me from getting inside and the darn bury move with it’s I-frames is ridiculous
This sounds like you are probably making your intentions a little too obvious. If you are getting beat by headbut or dashback Ftilt you can try baiting out those options. If he is shielding a lot you can use either shieldbreaker or tomahawk grabs. If he starts spotdodging you can use dancing blade. You can also try to dash at him, do a rising neutral air on his shield then drift back outside of his range. The best part about this is that you can react to any option he does reflexively. If he tries to Uspecial OOS then you can react with shield, if he shield grabs or Ftilts then you can Fsmash him. You can also try empty hopping just outside of his Ftilt range to apply pressure without getting in his face. You shouldn't overuse any of these options but you can use a mix of them (along with attacking him directly) to keep him guessing. And you can also double jump before landing to vary your timing. Also, you should get in the habit of spot dodging after falling aerials.
I'm also conveying that DK doesn't move fast enough when he B reverses, it's reactable, and therefore it's an option that works if the lucina player messes up, not an option that gets DK out of jail.
B-Reverse for down b and neutral b are very good for him and side b giving armor is nothing to be ignored since all of these options heavily punish Marcina if their juggling is misspaced or mistimed.
TBH you guys are both kind of right but I think you two are missing the big picture. B-reversing is good as a mixup but it isn't a get out of jail free card. When I juggle DK I prioritize Utilt and use Uair as a mixup. The reason for this is because committing too early with a jump gives the opponent the opportunity to either airdodge away or to reverse his momentum somehow. Dashing underneath him and sharking with Utilt gives you the opportunity to get a feel for what he's going to do and if he burns one of his options early you can react to it. Plus, Utilt beats nearly every landing hitbox DK has. I use Uair mostly as a surprise option so the opponent doesn't only expect one thing. Like, if you only Utilt then he can just neutral airdode through it then grab you or he can land on the top battlefield platform. So yeah, B-reversing isn't foolproof but if you know that your opponent is capable of it then you should keep that in mind. In general, I try to get at least two or three extra hits on a character that has a hard time landing. It's pretty hard to juggle indefinitely but two or three extra hits per juggle add up over a game so I think it's a pretty good rule of thumb.
Lastly, DK has a HARD time getting off of the ledge as well. At low percents, even a character with a bad recovery can do enough mixups to make edgeguarding less likely to be successful. Between 0-80ish% I ledgetrap and make a mental note on what my opponent favors when he recovers, and at high percents I go for the edgeguard (especially if he gets sent really far out).
EDIT: So I actually started playing DK a bit online and one of his major weaknesses is that he has ATROCIOUS out of shield options. On Ultimateframedata it says his best is spinning kong at frame 7 but that is the air version specifically and that one hits really high so it has a good chance of whiffing if he goes for it. Plus, the air version doesn't kill or really connect as well as the grounded version so it's too risky to ever justify using. The grounded spinning kong can kill but doesn't come out until frame 19 so it is really only good for punishing laggy moves. His next best one is Bair and it hits very high so it tends to whiff. Now, Marcina are pretty tall so it might not whiff against them, but it is still a pretty committal option that only works sometimes. Shieldgrabbing is really the only option DK has. As long as you space your landing aerials or buffer a spotdodge after you land then he can't immediately punish you for hitting his shield, he has to accept your shield pressure and whatever mixup you want to do. Now, he can preemptively Side-special or Uspecial if he thinks you are going to attack (like you mentioned), but these are hard reads that can be easily baited. You can jump at him and double jump right before landing (like I mentioned before), tomahawk grab, or just land in front of him and either spotdodge or dodgeroll.
