How hard is it to write/publish a 300 page eBook in 2-3 months on Kindle?

My answer would be “not very difficult”, even when you work full-time, but there’s two secret ingredients. Firstly, if you need to write fast, you will need speech to text software. Secondly, you will need to aim to get this book adequate, not perfect. You’ve heard of the 80/20 rule? It means that you can get 80% of the result by doing 50% of the work. Doing the next 20% to complete the project will take you a further 50%. So if you’re serious about this venture, decide early on what level of quality you are aiming at, what you need someone else to do, and what you’re prepared to leave out.