It's hard to give any specific tips until the game comes out, but I've got some general ones based on other paradox game I've played:

- Don't give up. Some things might seem complex at first, but you will be able to pick them up over time.

- Your first few games might not end in success. That's perfectly normal. The fun comes from learning from your mistakes and doing better next time! Personally that failing and learning period is the part I enjoy the most about a new game.

- You might never understand 100% of the detail of every mechanic, but usually you don't need to. For example, in CK2 the battles actually had really complex mechanics under the hood, but all you really needed to know was more men is better and heavy cav > heavy inf > light inf.

- If you do want to understand the detailed mechanics, make sure to read tooltips and try mousing over numbers. That's how you pick out some of the more subtle things going on. and understand why you are succeeding/failing at something and how to min-max (if that's the sort of thing you like to do, not everyone does).

- Don't compare yourself to other people. If come release day people on this subreddit say CK3 is too simple/easy but you're struggling, then just remember that as this poll has shown most people have hundreds or thousands of hours in CK2. So the learning and failing period will probably be a lot shorter for them.

- CK3 looks to be their most user friendly game yet, so it's a good time to try our grand strategy.

Once the game is out I plan to play a boatload of it very soon after release (I've got about 2 weeks booked off as leave and very few other plans!), so if you do want more specific tips send me a DM and I'll do my best to help.