First time onebagging for 7 nights

Hello everyone, I am a new onebag redditor, planning a trip to Spain. I'm flying with Ryanair and I'll be gone for 7 nights, so I bought a travel bag fitting the exact Ryanair requirements (40×30×20cm).

This is going to be my first time travelling with just one bag and I wanted to ask if anyone has any advice for me :).

I figured packing t-shirts the "army soldier way", is one of the essentials, but I was wondering what other hacks you might have for me.

I am also currently crocheting myself a travel pillow that could be filled with clothes, but I noticed that some people don't really recommend doing this, as it could get me in trouble. However, I'm kind of confused by this, as in why should Ryanair be concerned whether my pillow is filled with pillow stuffing or clothes, as it makes no difference to them?

To be safe, I could theoretically sew them in, so it wouldn't be possible for the control to check the inside of my pillow?

Im not sure so please, if you have any experience with this or any piece of good advice, let me know :)

Thank you and I wish yall a pleasant day ^^