I've done a similar trip across Southeast Asia with just a 30L backpack, and it was a liberating experience. A 30L pack can definitely be enough if you approach packing strategically. First, prioritize versatility. Choose clothing made from lightweight, quick-dry materials which allows you to have interchangeable outfits and cuts down on laundry waits. I brought a couple of long sleeve shirts and pants for destinations where temples or mosques required modest dress, and they doubled up for cooler evenings.
Packing cubes were a game-changer for me; they help compress everything and keep your pack organized. Consider rolling your clothes instead of folding them to save space and minimize wrinkles. I also swear by carrying a lightweight packable daypack—it doesn't take much room and is perfect for day trips or keeping essentials handy on longer travel days.
As far as brands go, I've had a great experience with the Osprey Farpoint 40. It's technically 40L, but as long as you don't overstuff it, it's usually acceptable as carry-on for most airlines. It's incredibly durable and has good compartmentalization. The Tortuga Setout is another solid choice, offering excellent organization.
One thing to keep in mind is the climate; Southeast Asia tends to be humid, so packing moisture-wicking fabrics will keep you comfortable. A compact, microfiber towel is a must—it dries quickly and packs small. Remember to include a basic first aid kit and any medications you might need, since pharmacies aren't always easily accessible or might not have familiar brands.
For further packing tips, you might want to check out minimalist packing guides by travel bloggers like Nomadic Matt or OneBag. They provide excellent insights and packing lists, often based on years of travel experience.
How has your packing strategy evolved over time? I'd be curious to hear if you've already tried some minimalist approaches before embarking on such a trip.