From my experience, system design is not too bad if you have practice building systems.
You may read some of the interview preparation books to get an idea of the concepts and keywords the interviewer want to hear - but it's not too dissimilar to what you would do in practice.
System design correlates to architect level work.
Leetcoding instead requires you to know the basic patterns to use to solve the problems and either A. be a incredibly quick thinker or B. have memorised tons of exercises - especially if you're not super young and age and sleepless nights are starting to catch up with you.
Leetcoding doesn't correlate to work in any way.
You just suck it up and spend 2 months preparing for it so you can have a nice looking CV and a truckload of money after you get the job.
I would weigh carefully whether you want to go to a FAANG though. Some people lucks out and they end up in a role where they do very little, others end up overworking.
You can make decent money contracting for less demanding companies (both interview and work-wise) and being tax smart (I would say almost as much as a FAANG, considering half goes in taxes).