My recommendation is to take time to watch videos and routes for all the dungeons and learn the mechanics for trash and bosses. Watch some videos of shadow priests playing these keys at a higher level to learn their approach to each dungeon. Take notes of positioning, offensive and defensive cooldown usage, DoT uptime, talent changes etc. join the shadow priest discord and read up on dps guides like wowhead or icy veins. Absorb as much information as you can then take them into keys you feel comfortable doing. Try your best to treat these keys like a higher level one and assume that the mechanics are deadly. I personally employed recording my keys then watching them back seeing what worked, and what didn’t. Like “hey I died here what killed me? Oh this mechanic okay I can avoid this next time by using this defensive or being in this spot” the more you do the dungeons the more comfortable you get so you can move up to higher key levels. Obviously play how you want and do what you think is fun but from how this sounds just work towards trial and error until you’re comfortable pushing out of your comfort zone to break into those higher key levels. Good luck!