Tough one- the uke isn't a particularly loud instrument.I have a loud voice, and when I perform I play into a single mic that picks up both voice and uke, so I need to generate quite a bit of volume to help match them up.
For me my index finger is louder than my thumb, because I have short nails and so my thumb strum is using flesh and my finger strum is using the back of the nail. I can get quite a bit of volume without flailing my arm, I tend to play with a fairly short, wrist action strum.
It's certainly not traditional, but I also often use thumb and finger picks, and can also strum with these as well as fingerpick, though it takes a lot of getting used to.
As for your uke, it should be as loud as most others, certainly the Mainland's I've played have had plenty of volume in them. The only adjustment I can think of is action height (as in bridge saddle height). Most players, me included, like a low action, but a higher action not only allows for more aggressive strumming before the strings start to rattle on the frets, but will in itself increase the volume. It's no coincidence that my loudest uke (the smallest-bodied one I own!) has the highest action.
It's also just occurred to me- how are you holding the uke? If you're holding it without a strap, clasped against your chest, you could be impeding the vibration of the top and back. Try resting it on your lap and strumming it whilst it's away from your body and see of that helps. Then, of course, you have to figure out a way of holding it whilst you to get that effect! Sometimes, just holding it a little more gently will do it.