As u/runtojakku said, look for breathable fabrics. I’ve had the best look searching for terms like tactical, utility, hiking, golf, and mechanic. Definitely read the reviews as well, they usually help clarify both sizing and the weight of the garment.

I really like the H2H brand on Amazon. Their products don’t look like most American clothing, which gives them an off-work feel. Maybe a little hot for April, but this Knitted Pullover jumped out at me. They also have a great selection of henley’s and raglan tees, which will be much cooler. Note that many of the H2H products will have several different styles bundled into one “product,” so look at every color option. The quality is pretty good for the price point, too. I've had one of their hoodies for years and it's held up really well for $20.

Givon is another brand that carries a lot of similar, or even some of the same, products as H2H. I haven’t purchased any of their products yet though, so I can’t comment on the quality. As with H2H, make sure you check all the product colors because they often bundle similar styles together in a single product.

The biggest issue I see with finding pants is the color, at least in terms of tactical or hiking pants. You may have better luck with golf pants. Alternatively, go with basic chinos or try for modern cut, straight fit linen pants.

As for scarfs, I really like this Explore Land tactical desert scarf wrap. At roughly 42 inches square, it’s big enough to simulate montrails. But it’s also lightweight and breathable, even on 80+ degree days in full sun. I originally got cooling towels to wear underneath it, but I haven’t needed them yet.

You mentioned that you had boots already, but take them out for a walk. We did 16,000 steps when we last visited Batuu and I was definitely feeling my boots afterward. If you don’t think you can handle that in your boots, try them with a good pair of walking or running insoles. I prefer something with good arch support to a cushy gel pad, but get whatever is comfortable for you.

With the belt, you might luck out and find something on Amazon but Etsy or a costume/cosplay store will probably have better options. I believe I’ve seen listings for the classic Star Wars belt buckle from several Etsy sellers.

I hope this helps and good luck building your bound. Definitely post a holoscan from your time on Batuu. May the spires keep you.