My mom took care of my first born for her first year and it was great. When the second was born three years later, I got full-time care for my eldest and then she took care of my newborn for a year. Although she would have done taken both full-time, I felt it was too much for her and too much to ask of someone and she would never have told me if it really was more than she wanted to take on. After the second child's first birthday, we got an au pair and my mom provides regular back-up care for both kids for sick days, vacation days, au pair vacation days, and also for date nights and long weekend overnights. It works great for us because we haven't burnt her out over the week, so she begs us to give her the kids more often - date nights and twice yearly long weekends away we have found have strongly improved our marriage.
I think we've found a great solution that works for all of us - we have reliable care in the au pair plus back-up care with my mom, the kids have their regular routine with their au pairs, who they've loved dearly plus lots of time with a really engaged excited-to-see-them grandma, our au pairs have had lots of time off and a flexible schedule so they can travel and take classes and get the most out of their year, and grandma has the kids at least once a week but can still have her own life and not be totally exhausted from being some child care provider for two kids.
Just our take

hope you find something that works for you!