I think it was Alex Rawlings who said that after your 5th language you start to understand what works for you and what needs to be done to acquire a new language. I personally adopted a very lazy approach which consists of vast exposure (through films, novels etc.) to the language with little additional effort. It works for comprehension but it’s not too great for active production. For that you need to study grammar and polish details. In fact, it might be easier to learn 3 languages to a mediocre level than to master a single language.

Maintaining language may be harder than learning a new language. I think I’m forgetting old languages faster than I’m learning new ones. You can only use so many languages a day so your experience tends to spread out like butter...