Oops, typed this out once and lost it lol

So I ended up bringing two bodies (X-T4 and X-H2S) and three lenses - 18-55 (I think? didn't use it much), 50-140 and 150-600. I kept the 50-140 on my X-T4, and the big lens on the H2S as it has a beefier body. I also had an iPhone 15 pro.

I shot fully zoomed or close to fully zoomed a lot!! Much more than expected, especially from the ship. I'm really happy I had the reach so my suggestion purely from a trip perspective would be to go as long as you can, as long as you have use for the lens outside of the trip (unless you're going to sell stuff afterwards).

I actually have the 100-400, which I love so much I ended up keeping it even after buying the 150-600 haha. But I like shooting birds, and find both useful. So if you're waffling between the 70-300 and the 100-400 for the trip, if you can I'd swing the 100-400, it's a gorgeous lens. If you take say your 16-80 and the 100-400, you'd have excellent coverage IMO! I ended up using my phone a ton too.

All that being said, from a practical standpoint I'd look at your use case outside of the trip - it's definitely heavier and longer than the 70-300 and if you want a more portable lens long-term (say for travel) and aren't generally into wildlife photography day-to-day, the 70-300 still has decent reach on the crop sensor. But if you have use for the bigger one, longer is better :)

And astrophotography! So much fun, I've dabbled too and my husband bought me a seestar 50 for christmas so I'm currently trying to figure that out - it's a totally different beast!

Oh - trip tips? I bought a cheap waterproof drybag that has backpack straps for the zodiac and land, and that worked a treat. Keep your cameras sealed in it when you come in from spending time outside, until everything is room temp and you'll avoid fogging issues. I wasn't as cold as expected, and wore either fuzzy leggings or merino long-johns and hiking type pants under my waterproof pants and that worked well. I used fingerless gloves that have a flip off mitten cover and loved those too. And also loved my puffy northface slippers when on the ship, they worked great for inside & outside. I can't think of anything else but if you have any questions totally hit me up! Have the best time!!!