I spent a couple of hours downloaidng house mods to fill up my world, but there seems to be a limit, any way to increase this?

If you have merged everything you could and are still over limit, time to start merging mods.

There is also a ghoting feature in Wrye Bash but I do not understand it enough about how it could help. You may want to look at the readme for Wrye Bash to see if that may be a solution for you.