Custom build script problems

So I’ve been working on a custom build script but I’ve run into so many problems. I thought that if I just made a gamemode with the base of sandbox I could basically do whatever I wanted to just “add onto” sandbox. It seems like I was wrong because at one point I couldn’t spawn anything, then at other times I couldn’t use setteam, and then I wasn’t sure if I could even have custom teams without messing up the scoreboard, and I could never use tools. All I really need access to is simple things like overwriting the default HUD with my own and then maybe some more screen elements like a custom scoreboard and player nametags. I would like to make this its own simple gamemode though instead of just making several addons. How should this be done in order to add my own custom content, yet keep the basic sandbox features?