For a personal Google One subscription, prepayment is definitely not available. You can be billed monthly or annually. Naturally, this is "pay in advance" so if you pay on October 1, your subscription lasts through October 31.
A few months ago, I canceled my subscription, and when I tried to re-enable autopay for it, I found that I could not, even though banner messages claimed I could. It turned out that I could not, in fact, re-subscribe until after the subscription had lapsed on its expiration day! I had no trouble doing that, and now, autopay is re-enabled for next year, but it sure was stress-inducing to just watch it expire.
A few months ago, I canceled my subscription, and when I tried to re-enable autopay for it, I found that I could not, even though banner messages claimed I could. It turned out that I could not, in fact, re-subscribe until after the subscription had lapsed on its expiration day! I had no trouble doing that, and now, autopay is re-enabled for next year, but it sure was stress-inducing to just watch it expire.