Hello and welcome to the community! While I am a huge fan of Half-Life and playing the mods and know a lot about the game, I still have yet to actually delve into making mods for the game. I still plan to do it, but I have only been designing mods myself for the game Battlefield 2, and am working on my 3rd and most ambitious mod which is a historical total conversion called Battlefield 1944 a true HD recreation of 1942 in the BF2 engine with up to 4k textures, and ported maps and historically authentic weapons and content all in HD pushing the limits of the engine. Basically making the game Battlefield V should have been. Now that BFV has had all support for the game cancelled now it is all the more important for us to finish this mod.
Also shameless plug:
https://www.moddb.com/mods/battlefield-1944
So for that reason alone I still have not had time to focus on modding HLDC or learning all the intricacies of the engine yet. What I can tell you now off the top of my head, is that apparently it is all drag and drop as far as porting files goes. You can only use either early mods that were made before the official mod tools were made, stuff with simple map and skin mods such as They Hunger and stuff that never used custom .DLL's, or recreating mods. For this reason there is no way to really get super advanced mods. So if a mod uses custom .DLL's you will have to break the mod way down and remove a lot of scripts and things dependent on the .DLL. Paranoia was ported and worked well that way.
You can also optimize the mod for better performance on the DC in a few ways. First off you need to convert the textures of everything to .PVR format. This helps render the textures in the DC's native format. Another thing to keep in mind is that the DC has only 16mb of memory, and as such mods with larger levels need them to be cut down in size to fit in the systems memory. So keep this in mind you may have to edit and link parts of maps into 2 separate .BSP's.
Most of the guys who worked on the mods such as Ovverride, Dreammatrix/R4mbo and cptPauer are long gone from the community. Dreammatrix I spoke to about a year ago, he was still around and discussing making more mods but he is a very busy man. Hasn't been on the Dreamcast-Talk forums under his R4mbo alias for over a year now sadly. So getting tips from him might be hard to. cptPauer on the other hand, I spoke to about 4 years back asking for his 4X4 Evo mod he created and how he was converting the PC LTE tracks to the DC version of the game. He was still active on his Youtube channel.
www.youtube.com
Maybe it would be a good idea to contact him and ask if he has any tips and tricks for you? He was still uploading videos to his channel as of 5 months ago.
I really hope you can get into modding the game on DC! I for one have always planned on working on a WW2 mod since the assets already exist from Day of Defeat, and then maybe making a singleplayer Halo game since somebody ported some Halo CE assets into a MP mod, it could be possible to use those to put something together that was fun and would give us a Halo inspired game on the system.
Wishing you the best of luck!