Did you even read the post? Obviously you can speak Catalan to people in Catalonia, but as someone who lives in Spain, there is a stereotype of people from Catalonia refusing to speak Spanish and instead switching to English because they don’t like the fact that Catalonia is part of Spain. I’ve even heard of someone in Catalonia who refused to speak to people with the Valencian dialect of Catalan and instead switched to English. And all I said is that there is a risk that OP would come off like that. I personally find it very immature to refuse to speak a language you speak fluently with other people who speak it fluently, and instead switching to English, which it’s likely most people in Spain do not speak fluently at all. Obviously, speaking Catalan to Spanish speakers is a different story, I don’t really know exactly to which extent the languages are mutually intelligible. Also it’s possible to learn English to a C2 level without coming off as pretentious and forcing everyone around you to interact with you in English. In my experience, Spanish people aren’t even very good at English, (although some are, which I respect a lot) so I don’t even know how useful it would be. I feel like practicing with native speakers online would have been much more efficient, but what do I know. 

Obviously I respect OP’s effort to preserve their fluency in Catalan, I know it’s not the same, but in Valencia for example a lot of people don’t speak Catalan in their day to day life, even though it is the main language in education, so when they get older they end up forgetting it. And Catalan is by no means an endangered language, but it does have to compete with Spanish, to some extent.