Using a sling vs. a packable day pack for travel

Hey all! I'm finally starting to travel, and I'm new to the world of one bagging. I'm going on a couple trips soon that are about a week each that will involve all day exploring. Typically I'd throw everything in my pockets, but recently I developed an allergy that requires me to bring two epi pens everywhere. I also have some other small medicines and knickknacks to bring so I thought I'd invest in a sling.

I bought a Bellroy venture ready sling 2.5 which fits all my things tightly, but when trying it out it's been uncomfortable. As a crossbody my shoulder gets sore. As a waist pack it was more comfortable, except for when walking it feels like it's constantly slipping off. Now after researching more I'm exploring other options like packable day packs to bring in my carry-on that I can use on my trips.

Overall I value:

• All day comfort when walking around. My goal is to be able to forget I'm wearing anything at all.
• Being airline compliant
• Having some basic style (I just don't want to stick out in a crowd, but overall comfort is most important)

Sling ideas:

• CODEOFBELL X-POD II - Pros: Looks like it won't slip as a waist pack and has a supporting strap for cross body. Cons: expensive and looks a bit too tactical

• MODERN DAYFARER ACTIVE Sling - Pros: Looks nice. Don't know how comfy it is though.

Packable day packs:

• MYSTERY RANCH In and Out 19

• AER Go Pack 2

I would appreciate any suggestions!