I've heard that "merging" mods is an effective way to save some space in the .esp folder, but I have no idea how to do it. Could I have some advice, please?

There are lots of answers out there: https://duckduckgo.com/?q=skyrim+how+to+merge+mods&t=ffsb

Merge script plugin for Xedit, theres even a standalone .exe version that'll do it for you one of the top mods of last month on Nexus iirc