I actually prefer crash bomb by a significant amount in the matchup, but that may just be preference.
I feel like I've posted a lot on this particular matchup, so I'm going to approach it from reverse.
consider the matchup of Mega Man vs Rosalina without Luma. Rosa seriously has nothing advantageous against Mega Man without the little guy. Mega wins the neutral, the air game, the ground game, and the killing game.
Got that? ok so now your focus should shift to "the goal of this matchup is to remove Luma from play and keep him out of play". I can't believe how many people I see play this matchup without this mentality... and I'm not just talking about Mega mains.
Ok so removing Luma. Mega Man is very good at pushing Rosa backward slowly... even in your post, you mentioned 2 retreating options as good play for her. this is the natural progression of the matchup. you chip away in the neutral and force her back closer to the edge. Once she's there, dtilt, bair (3rd hit), fsmash, dash attack (last hit) all knock Luma off the stage early. dtilt in particular will hit him off the stage at full health, will hit thru shield, rolls, and landings, and will only be punished with a Rosa grab usually.
So I've outlined that Rosa will retreat to the edge fairly often, and Mega has good tools to remove Luma. That gives you 12 seconds of like a 7-3 matchup against SoRo. And to add to the pain, if you learn the timing for Luma coming back, you can bair him before he acts to (potentially) kill him again without Rosa ever being able to use him.
Focus on killing Luma any chance you get and this matchup becomes significantly easier. you can even juggle him with bairs to move him from center to the edge.
Now this story changes a bit if Rosa chooses to separate Luma at the beginning. This does 2 things... (A) it makes Luma easier to hit without fear of punishment from Rosa and (B) it makes Rosa easier to hit without fear of punishment from Luma... honestly take your pick at this point... both are acceptable answers.
In neutral, there's absolutely no reason to not spam Crash Bombs and watch for her to SH Nair over one. Her DownB doesn't hurt us so it's a reset only, and you are training her to downB in the midrange, which will help on a bait later on.
SH pellets work as normal, and stop Luma from hitting you if you shoot him, so that's not terrible. I don't pellet much in this matchup though. Full hop Down45 metal blades are really a win win for you. If they hit, you get a grab or attack (utilt/usmash) followup. If she shields, you can generally get a grab or just bair the back of her shield. If she downB's, you get your choice of attacks.
When edgeguarding her, she has a very adaptable recovery, but it never puts you in danger (as it doesn't hit you) as long as you don't overreach. keep throwing projectiles and fish for landing a bair on her trajectory. If she gets directly below the ledge, dair will end her.
Conversely, she's very good at edgeguarding us. Be adaptive. Go VERY low sometimes. burn your upB first and go VERY high other times. keep mixing it up and try to make her commit, then go another direction.
When you're in the air trying to land, just fade back from her and nair... no need to challenge her shield.