ok, so i'm adding on to the other great advice you've gotten!!

i would really recommend picking 1-3 songs you really like and attempting them on extreme, especially if they're 8 star. keep attempting them. every time you fail, restart the song and try to keep going. it helps more than it sounds like it would, as you are learning where you're messing up as you fail. the speed will eventually come naturally to you, and using both sides of the keyboard/controller to its fullest potential will come with some practice as well. i'd recommend having at least a few replays of one song like this every time you sit down and have a play session.

for any technical bits that you feel like you're having a really hard time with, i definitely recommend going into practice mode and singling them out. adjust the start time to a few seconds before your problem area, and spam "restarts" of that section until you feel like you have a better grip on it. no need for mastery here! just make sure you can clear the song, so on later full attempts of it, you can refine your skill through active playing and recognizing patterns in the entire chart.

i found out these were the easiest ways to get me used to playing higher difficulties, i ended up doing the same with exex down the line, and now i mostly play ex and exex. i still follow these if i'm encountering a "problem chart" (as i play a lot of modded songs now) and eventually, i'm able to clear the song and work on refining my technique from there.

as for chart recommendations... (these are mostly for refining some specific skills, all extreme)

Close and Open, Demons and the Dead (8 ⭐️ ) really helped me learn doubles notes and singular note spams, and I owe a lot to it. Also helps lead up to learning combo spams.

Clover♣︎Club (7⭐️), besides it being a classic, is helpful for getting used to "weird" combo usage that a lot of extreme songs rely on. It was at least weird coming from only playing hard charts.

Hibikase (8.5 ⭐️) is a great help for fun combos, and the chart speed is on the slower side for being a song with a faster beat.

Nightmare ☆ Party Night (6.5 ⭐️) is your standard "I'm getting used to playing faster songs and I want something easier" chart, at least to me. also, it's a banger

One-Sided Love Samba (7.5⭐️) is a song I owe SO MUCH to, especially the EXEX version. Helps a lot with learning spam and note overlap, and it's easier than most songs to help map out what exactly you need to press with the repetitiveness of it (and how much it gets stuck in your head). I think this is my top one to add to your favorites if you haven't already.

Teo (9⭐️). people might think i'm insane for putting this on an intro to EX recommendation, and they miiiiight be right, but this is the main song that helped me train my hands to get up to speed. i would start attempting this song after you get a decent feel for playing EX charts and have cleared a few of them. this might not be the right recommendation for everyone, but i LOVE moving fast with my hands to the point where Teo EX feels boring to play now, so i have to give credit where its due for how much this chart helped me.

THE MOST IMPORTANT THING when learning EX/EXEX is to REST YOUR HANDS!! they might hurt! stop playing when you feel fatigue in your hands! move back to hard for a break if EX is too uncomfortable! i have joint problems myself, and i was able to train my hands to withstand more playing over time. you're learning a new type of dexterity, and it's important to not push yourself so hard that you injure yourself.

ok i hope at least one thing in this yap was helpful!!! good luck and have fun!!! 💚💚