For 1 week, write down everything that you do for your company, and then group the like tasks in order to figure out how many/what type of person(people) to hire/train. Start looking for that person, while writing down everything that you do the 2nd week. Rinse and repeat as you go through the month/quarter, and you should develop a better idea of what it will take to replace you.
The LOI writers know that you are doing the job of multiple people, and that you are indispensable, and that's why they want to require you to stay. Make yourself replaceable, and then there would be no need for you to stay. Work from a beach if you want, but take action now to permanently lessen your stress.
Feel free to reach out if you want to talk (or just vent) privately. You can find me through info in my HN profile.