Thanks for asking and sure thing! I will give you my suggestions that can improve the game overall. I think you can approach this rule thing from a different perspective, remember at the end, the game you want to make is fun for players to play, easy to understand, so players who play the game will have a good time playing,
For most or if not all players, we all know the game revolves around the traditional sense of playing Uno (which means players are going into the game with that understanding), but this time, the player is a card shapeshifter. I feel the game can be more fast paced a little bit personally, as I had to go through many rounds, for the first round I guess I was just very lucky and did not feel like I have to switch as it wasn't my colour / number and instant win. As I think traditional Uno is a game that takes a very long time to finish, so I think finding a mechanic of some sort to make the game go through quickly will be ideal, either shorter timers. Do implement cards like Skip and Reverse, as this makes the gameplay loop very interesting as you never know when will be the player turn. I don't think you need to add the +2 and +4 cards, as it will prolonged the game (maybe you can and the AI can try to ultimately bomb a player to get him off the game by getting over 21 cards?). I think you can add the card which picks the colour for the next person which (perhaps at a higher chance deliberately targets the player). This is to make the gameplay much more intense, interesting, as traditional Uno is very slow paced. Slow paced is because as the player with many cards and players have to be strategic with their cards, but as a card shapeshifter, I think there's not really much thinking but the rules and trying to survive and not to get picked, so I think of this game mode straying away from traditional uno, to be a much more intense gameplay, to make it fun for players to avoid waiting for other players turn and the game prolonging for long where players do nothing but wait.
I think you can already start the first round with an easy rule already so that the players do not straight away associate the game with traditional Uno. I think the rules can be implemented before the start of each game, I think of Hades when I can pick which perks to make my game harder (like speed running, more enemies) but in this case, the game will pick the rules for me, and at least me as a player I will be aware of this rule, example, any number that is 5 is wildcard (which means can be placed no matter what). If the player dont remember this rule and the traditional Uno rule, they will lose.
You then can scale up the difficulty, by increasing the number of rules. X colour and X number will be wild cards, or the number 6 is equal to 9, the colour red can be used on yellow, to make the game very interesting and fun. You have all sorts of combination that you can randomize or allow the players to choose. Higher level means higher difficulty which means more rules that the player have to keep in mind.
Of course these are just ideas and suggestions which would be pretty neat to include, that your team can bounce of from in terms thinking for the rules of the game. Definitely letting the players know the rules beforehand, rather than finding it out during the game will be so much better, it is the same with any traditional games that like to deviate, we will always set the rules before playing, and not change it in the middle of the game.
Really sorry for the long essays but I do hope it helps and give some insights on how you can further tackle and improve the game! Please do not feel obliged to take my suggestions as well as this is just my experience and how I see the game. Really would love to see this game expand as it has potential to be more than what it is now. It will be cool to see different characters and AI playing the game, maybe some quicker, some slower? Think of ways to throw the player off the mindset of a traditional UNO game and make sure it is fun!
EDIT: You can also include a hidden rule that the players will speak of it, but I think this should be seen as a fun rule, like you can only play red at the end or something ahaha, or some other fun rules that makes the game fun.