The API documentation itself is kinda confusing, but it mainly says that you have 100k requests per minute for the entire game, not per server as a lot of people think it is.
For sure there is a limit on how many requests you can do on a server per minute. I recommend you use the APIs just like if it were a data store.
If you do SetAsync using data stores multiple times in a short time span you will for sure run into issues. The same happens with MemoryStoreService and according to what I understood from your post, you are overriding something in MemoryStoreService every second.
Correct me in case I understood your post wrong.