Hey there! New-ish amateur level designer here so please take anything I say with a grain of salt.
I love the atmosphere of Black Lagoon and from my point of view, your levels seem to have a consistent and solid quality to them. I noticed the Gmod level has a video but it would be cool to see it being played through so I have a sense of how it works within the actual game. Really for any of them I think somewhat of a playthrough would be a welcome addition. Also the Black Lagoon page doesn't have a download link.
One thing that could help draw attention is making taking a screenshot of the store pages w/ the ratings since all of yours have 4/5 or better. I think this could be a good way to show how the community received the map and sort of show off your commitment to quality in a way.
And then as you mentioned maybe showing more of your process of designing and testing the maps, along with the iteration/changes you've made would be a nice addition as well.
Overall I think as a hobbyist I think you've got a nice portfolio. One thing I have heard people mention is to sort of separate your newer/better quality creations from your old ones. So if there was a way to have 3 of your best maps in one section, and then older/less relevant stuff on another. That way people don't get overwhelmed from the amount of maps, and they really only see the good stuff.
Again, keep in mind I don't have much experience to speak from but these are things I've heard others mention, or just what I think might be a good idea.
Overall great job!