I am just an hobbiest myself and just like you trying to find my entry as a Level Designer somewhere. For me personally its multiplayer LD what drives me and since I found Mapcore and became part of the community I learned a lot from the people and experiences. I think, being part of an community, playtest with them, talk to them and giving them feedback really helps you develop as an Level Designer and person. Because it is not just about the LD skills but also about people skills like communication, recieving and giving feedback, presentation and etc. Working with other people (like environment artists) also helps a lot on developing these skills and shows you can work together with other people (which is required in a studio).
I think it is good to not just do LD, but also experiment with doing some environment art and finishing a project from start to finish. Because this means you have to plan/schedule a project from start to finish. It helps you understand what a artist will be up to, so what he will recieve from you as an LD and how you would translate a greybox to a arted finished map (gathering good references, working on interesting geometry and composition). Also you will get trough topics like optimisation and polishing which is something for later, but you should already think of at the start of doing certain maps (especially in Source :P). And other than that, having finished good looking pieces on your portfolio will defenitly get their attention! even though you just did the Level Design part. And who knows, maybe you find out you like environment art more afther all or both! Maybe joining a mod team somewhere is something for you, it is a interesting way to work with other people from different disciplines.
As Radu was saying, he convinced me on doing a map for FC5 and helped me on giving some direction like he would be my lead LD and I report back to him. This basicly helped me plan and document my project like you would do in a professional setting. I am not sure what is the best way, but I think its about showing what decision you made and why you made them during your project and what direction you plan to go with your project. Draw conclusions from your research and player feedback and show how you changed course towards the right direction because of this information and perhaps what you learned and would do better next time. I don't think recruiters go in-depth at first, but when you got their attention and got a interview planned, they will defenitly dive deeper into these kind of things. Also I had to work with a different tool and type of game which was a great new (learning) experience. I would defenitly recommand you trying out this editor, it is really easy to learn and you can build full maps really quickly.
I think it would be worth it doing some Multiplayer LD. Mapcore is mostly focused around CS:GO, which is quite a tough game to build maps for as this game is super competetive. BUT I think you would learn a lot from it and would probably enjoy being in contact with other Level Designers. The easiest way would be playing some CS:GO and trying to get a good understanding of how the game works and why it works (if you didnt yet). Then try out some stuff in the editor, maybe watch some tutorials and such and see if you can come up with some cool layout ideas. The wingman gamemode would defenitly be the easiest to start with as it is more casual. There are two (discord) communities who provide playtests and feedback which might interest you.
Source Engine: https://discord.gg/KBT9MD
Oh and there is this topic: