So apologizing ahead a time for the long post, I started typing out a quick small version but it kept getting longer and longer of a post. I felt like each move deserves a longer explanation
With Roy you'll need to learn how to utilize movement, string combos, establish a grab game, and understand how to utilize DI for best recoveries.Roy can play really aggressively but requires a necessity to play smart at the same time. He has decent tools vs. most the entire cast. From my experience he performs worst against people that are good at high speed pressuring, those with projectiles, or people that can utilize crouch cancelling effectively. Some notable bad MU's from my experience include DK, Marth, Link, Mario, Pit, and sheik are a handful that come to mind.
L-cancels are extremely important, as they usually are with most any character in the game. Like you said Roy is typically a character that likes to stay grounded but has strong aerials to combo with. With that said, don't force wreckless Fair SHFFLs or anything like that. I used to do that alot and got punished all the time for it.
Detailed explanations
[aerials]
Fair- one of roy's bread and butter moves; use it to wall out opponents and control space. Some alternative ways to use it are standard SHFFL falling Fair, SHFFL Rising Fair while in place or moving back (this is more of a spaced, safer Fair), and full hop Fair to mix up the angle or create opportunities for Double Fairs/Double Aerial mix ups to platforms.
Dair- Typically used as a tech chase starter after Dtilt pop up combos. Sends them down with the sweet spot. The sweet spot is just about where Roy's armpit is. Only safe time you could attempt a meteor kill is against certain people's recoveries or tether characters. You can time it so that you can ledgehop Dair and pull the meteor off for a kill, but it can be hard to time right. If you input the Dair on a rising Short hop, the hitboxes can be really useful as well while trying to catch people's rolls. Be sure to practice the timing of a rising Dair SHFFL, i had a weird timing feeling out the timings for it.
Nair- A great move for controlling wide spaces. I like to use it as a combo finisher as it has lots of knockback on the second hits. It can be full hop fast-falled to surprise people on the second hit as well. I wouldn't suggest spamming this as it CAN be somewhat punishable. I avoid using this on shield as well
Bair- I only consider this a finisher and it's one of Roy's strongest kill moves if you hit the sweetspot (again, practically in his armpit. It's easier to hit if you're dashing into them)
Uair- Great move for juggling and pressuring people on platforms. It covers a good amount of space and can be used to juggle for a few hits. I like to chain a SHFFL Uair with Utilt to keep combos going longer. You can also vary the Uair hitbox depending on how fast you SHFFL. It has a hitbox that you wouldn't think would hit people but does in the front if you land really close, this if L-cancelled can be followed up. Also you can cross-up people with Uair and interrupt or combo start with Side-B.
[Smash attacks]
Fsmash- As you and every roy player knows, closer to the hilt is the strongest part. You can get some monster knockback on this and is probably is best kill move if successfully hit. Wavedash >Fsmash can be incredibly useful
Dsmash- I Don't use it a whole lot personally since i tend to get punished from the endlag. It has a great startup ( i think it's a 4-frame startup?) You can use the invincibility frames off ledgehopping to waveland onto the stage for a quick Dsmash if you do it fast enough. You'll see someone like Sethlon use this all the time
Usmash- Can be DACUS'ed. Can be useful, i personally almost never use it intentionally. Sorry i cant elaborate much on this
Dash attack- A good tech chasing move if you can catch the read. Also useful if you knock someone just off the edge and can catch them, it'll pop them up for a perfect aerial chase.
Jab- Jab i find really useful for grounded interruption. I like to throw it out when i want to get pressure out when they're in shield and need quick moves to try and catch them out. (dtilt>dtilt>jab>dtilt>Ftilt>etc)
[tilts]
Dtilt- The other bread and butter for Roy. This move is amazing and leads into most every one of his other moves/combos. Get familiar with knowing the timing of dashing and crouch cancel>dtilt and know the soonest time to do it. Dtilt has an ending lag that can seem long but can be cancelled out of by tapping forward to dash (knowing this you can Dtilt and quickly input the dash followed by immediately shielding, Fsmashing, or Ftilting)
Ftilt- Amazing for keeping space as well as prolonging combos and techchases. A simple Dtilt>Fair>Ftilt works wonders. Ftilt has an extremely long range and can be really useful against characters that rely on spacing like Marth.
Utilt- I like using Utilt as a 'keep the combo going' kind of a move, especially when they're above me on platforms. It's most useful to me in combos where you get off an Uair or two while they're on a platform and follow it up with a Utilt
[specials]
B- 'flare blade' you can use it to chase off stage for aerial 'pseudo smash attacks'. It has lots of knockback and comes out decently fast. You can throw mix ups in the air and change direction. Charging it up at the edge works well against certain recoveries that aren't as easy to sweet spot.
DownB- typically never used all too much. high risk high reward. Easiest use is for edgeguarding recoveries with hitboxes (fox and falco come to mind). Just know that the window is incredibly small but if successful the knockback is huge and the damage to counter does 1.5x the damage it was dealth with.
sideB- or DED(double-edged dance) There are obviously four hits to this move and the speed can be varies depending on the inputs. (holding forward b for all four would be typed out as X>>>, 'x' being the first strike since the first strike is always the same slash). You could do more digging and experimenting on the timings and variations. I personally can't give you too in depth of an explaination to the depth of this move cause i'm not a pro lol. However, useful SomeDED inputs are- 1. x^^, 2.x^^>, 3. x^>>, 4.x^>>.Know that the first hit never changes, the second up hit has a better hitbox, the third hit can be either a tech-chase(^), another comboslash (>), or a shield pressuring multihit (v). For the fourth hit you can either launch them up, knock them away, or have a powerful multihit burn.
UpB- self explanatory, just know that instead of going in the upper left/right diagonal you can input up>B>hold left/right to get the maximum horizontal distance.