The "You will need a booster in 3 months, 6 months, a year, etc" is nothing but fear-mongering. We don't know exactly how long immunity lasts because the vaccines have only been in people's arms for so long. We know for absolute certain that it lasts at least x amount of time, but it is likely far far longer than that.
The reason you will need to get a 3rd vaccine in the near future (2022) would be to account for variants.
As for being deathly ill, I can't speak on her account, but out of everyone I know that has gotten the vaccine, I've had the worst side effects (bed-ridden for a day with a 103 fever), but I would gladly do it again. Felt a bit worse than when you have a bad flu, but the reality is, you get to choose when you want to feel ill for a bit, and it's only for a day or so.
The last time I had the flu, I missed a whole week of class at a very bad time. When you get a vaccine, you can schedule it however you like and you get to choose when you will potentially feel ill.
Even then, it's only a small minority of people that actually feel ill to the point of inactivity.