Hey @ORTyOW! Welcome to the Forums!
Instead of having to merge all of your assets into a single mesh, what may work best is to take your modular house and decorations and create a blueprint out of them. Then you would migrate that blueprint and all of the necessary parts! When you migrate the blueprint it should also migrate any asset required by your blueprint. Like it says in the Migrating Assets documentation:
“The Content Browser contains the Migrate Tool , which allows assets to be copied from one project to another, along with any dependent assets. For instance, you can migrate a Material, and any Texture assets that define that Material will automatically be copied along with it. This is useful when you need to merge or fork projects, or when transitioning from a test environment into a production project.”
All you need to do is select all your assets you would like to group, then at the top of the viewport click blueprints and convert selection into new blueprint class.
4.x example:
5.0 example:
I hope the above solution works for you!