Play Persona 5 first since you already have that. If you feel like playing it again after then get Royal. It's basically the same game but with a lot of extra flair and some extra dungeons and an extra party member.
A lot of people tend to say it's good outright but it should really be up to you wether you want to spend 60 bucks again just for a dlc worth of content.
i suggest, you play 5, since you already got it, then if you haven't played 3 and 4, play em, since those are great games, then play 5 royal, (and replay 3 portable as femc if you wish)
