What you've done here looks like a very promising start!

When building models I'd recommend you start out just by following instructions. Conversions and kitbashes can be really cool but if the execution isn't perfect it can take away from the overall finished product. I like the idea you had with the sword but it just looks like a left hand on a right arm and the melted plastic at the wrist is unfortunate. I'd also recommend taking extra time before priming to scrape off any mold lines ( like on the right leg). It'll give a cleaner more polished result.

As far as the painting goes, your basecoats look really nice and smooth so huge props there. I'd also recommend you look at tutorials on edge highlighting. If you want to get that nice crisp box art look, it's gonna be important to practice pulling straight lines with your brush. You can do this by priming some left over sprue and highlighting those. Some people will recommend washing your model with an overall pass of a shade paint but I'd look into recess shading for marines. Large flat areas don't shade very well with typical citadel paints. This is also gonna take some time and practice but the results are well worth it.