Is learning related languages wise?..

I mean, of course it's better to know just ONE language at least on the Intermediate level than to study 3 and more, being a beginner in all of them. I still don't know English well myself, but I've become interested in Italian (for a very weird reason), so I'm trying to learn the language even though Spanish is much more common and "helpful" abroad (and French has too difficult phonetics for me; I already struggle with that enough in English). So, even though right now I'm a beginner and have to complete at least A1 level, it would be nice to try other romance languages in the future.

I'm a native speaker of Russian (but not Russian myself), so I've also been interested in other Slavic languages (tried to learn Czech to be able to study there for free, but stopped for obvious political reasons), even though I wouldn't be able to use them anywhere really. It feels like the likeness rather disturbs that helps.

I'm really interested if some people have/had been studying two (or more) related languages at the same time and what it was/is like?..