Adding extra luggage (musical instruments) to return flight only

Hi there! Asking here because I have tried this ad-hoc during my last trip and I kind of failed so I wonder if anyone has a better idea how to approach this.

I will be flying to São Paulo from Manchester, through Paris or Amsterdam. On my return flight I want to bring two musical instruments with me.

Last time I did this, I tried reaching out to KLM while in Brazil but I couldn't, and it was a bit rushed so I didn't insist again after calling and not making it through multiple times.

I wonder if I called them before even buying the tickets to quote this, or maybe tried to book the tickets via a booking agency instead of the app I could make an arrangement specifically for musical instruments? It's an electric guitar and an electric bass, both in cases, so nothing huge like an upright bass.

Ideally I would love to have them transported inside the cabin rather than as dispatched luggage, so my question really is how do I even approach KLM to arrange that? And most importantly, how do I ensure that when boarding/checking in in Brazil the "purchase" of these extra loads is honoured instead of me having to re explain everything? Really, any pointers is much appreciated, all I seek is ideas or previous experience with something like this even if on different routes.

Thank you all!