Title says all. Even on lightweight
mode. 200 mods, 10 GB of RAM.

some structures are massive, you can disable the ones you dislike which is what ive done. extreme lag is to be expected when the mod starts generating incredibly large structures. structures that generate mazes also consume tps so you can disable those too if you wish.
lightweight mode is generally for servers so you dont need the mod on the client, it doesnt do anything for performance, perhaps maybe a tiny bit of ram due to no blocks or items being registered. this should be off when youre wanting to disable structures ingame.

you do not need configs to disable them. for structures, stand in it and type /#whatisthis, to get the name,
then use /#edit structurename to then set it to inactive (not world inactive), do this for every large structure and maze structure causing your worldgen lag.
there is a config file for recurrent complex and you can use it to disable structures too but the complexity is far more than just doing the above.

I have no clue to how remove structures. No configs or files related to them are given.

Or you could visit the wiki page titled "Disabling Structures" ;)

Regarding the issue - this would be a duplicate of #155

/#edit is not a command.

it is if you have it installed client side and lightweight mode is set to false in the config