Welcome!! What parts of Pokemon do you like? The trading card games (TCG), the console games (too many to list), the mobile/switch games/apps (Champions, Unite, TCG Live, TCG Pocket, Pokemon Sleep, Pokemon GO, Cafe Remix, Quest, Shuffle, Majikarp Jump, Smile, Friends, Playhouse, etc.), the metchandise like plushies, the anime, the manga, etc..?
I am here to answer all your questions on my profile. It 's a bit hard to amswer there because you made each question its own post. No, you cannot change your name here on the Forums. It is directly connected to your Pokémon Trainer's Club Account and once that is set it's permanent. Champions is currently only available on the Switch but will be on the phone on the future. Free to download, so if you have a Switch you can get it immediately without a worry about money but if you don't have one you'll just have to wait for the phone version. The best expansion to open right now is a tough one... I can't find packs IRL anymore, so I don't know what the luck for them are. However, based on the cards in each expansion, I would say any of the Mega Evolution expansions. Mega Evolution, Phamtasmal Flames, Perfect Order. All of them after the Scarlet and Violet expansions. I just think they have some of the best cards to date. I didn't understand the Musa cousin thing, so please explain that to me. I hope this helped! (Oh! You also asked about playing with family and friends. You have to have a system for each person. When the phone's comes out that will be easy but right now everyone has to have a Switch. Also, you weren't a nuisance. Now I'm out!)