It might be monotonous to you but clearly he's onto something and enough viewers don't have an issue with it. Brazil has some of the most beautiful men in the world. I personally hope he gets to Colombia, Puerto Rico and Dominican Republic, which are full of beautiful men. (And Cuba and Haiti, when that's politically possible.) If he really wanted to mix it he up, he could go to South Africa, or India, or Vietnam, or Japan, to name a few other countries.
Working in Brazil and LA, he's been able to start his own studio, Voyr. For Voyr he and the other performers have sex with men of all races (he doesn't neglect having sex with Black and Afrolatino men, or Asian Americans, or non-Black Latino men, and he has scenes with White men too), he does really passionate individual scenes as well as group scenes of varying sizes (threesomes up to group orgies), and he mostly seems to be filming with gay and bi men. Not sure what you're talking about in terms of "heterosexual" men he's filming with, though there are self-described heterosexual men in Brazil and every other country who have sex with men, but I'm not sure this applies to RhyheimX's partners.
Same with Nico, who this thread is about. Nico is filming sex scenes with gay and bi men. In the scene I posted clips of above, Nico is gay, Boejack Ryan is gay, Marlon Costa is bi, and Will Duarte is bi. Not one of them is "heterosexual." In another recent scene featuring Nico, he and Roxas are both gay men, and their scene partner Yuri Leal, is a gay man. Lastly, RhyheimX does film a lot of sex scenes with White men. If he wanted to go Prague or some other predominantly White country, I'm sure he would. He went to France a few years ago and he got a career boost early on when he filmed in Germany for TimTales, but he appears to know what he likes and it's working for him. Maybe someone else can go chase the Bel Ami crowd.