If you want to achieve the authentic shamisen tone and experience, the bachi is an essential item. Interestingly enough, I wouldn’t necessarily consider it a “needed to play” item because practically speaking, you can still make sound by plucking the strings with your fingers, a guitar pick, or a range of items!
That being said, a big part of the shamisen’s unique tone is influenced by the bachi (as it can create both strong percussive and melodic tone at the same time), and so I would highly recommend getting one. The bachi’s large size does make it a bit awkward to wield in the beginning, and so whenever it starts to feel too uncomfortable to use, it’s perfectly fine to temporarily switch to fingers or a guitar pick. (After all, the most important element of learning is to keeping the experience fun and engaging. If the challenge becomes frustrating, that’s counterproductive to learning.) Eventually, your comfort with the bachi will grow and it will become your favorite plectrum!
Striking strings with the front of the fingernail is a great way to achieve a simple snappy tone!