For most people, kava gets better as you go. Beginners may not feel it completely and sometimes it can take several weeks before you do. Still, it's not a huge dramatic difference, and the same can be said for your questions about different strains. Yes, there are strains that are typically stronger...and sometimes different batches of the same stuff vary in strength, but, with that said, it's still never mind blowingly drastic of a difference. Noticeable difference, yes. Big difference, no.
Definitely try other strains, you'll find one that hits you stronger or in a way that you more prefer.
If you wanna pick some from the left, right and middle of the kava spectrum you could try:
Stone/Boroguru (strong & heavy & sedating) Mahakea/BoroNgoru (slight sedation, nice head buzz) Mo'i/Hinehina (clean & light feeling head buzz)