I did a bunch of this John’s Hopkins course and I remember it having a lot of exercises to use to practice.
If you are already really good at R it might be too simplistic, but it’s worth checking out.
https://www.coursera.org/specializations/data-science-foundations-r
They also offer this shorter intermediate course
I have had some fun using R with AoC, but that exercises skills that are rather NOT typical of the kinds of problems R is particularly good at. The suggestion to read Advanced R seems worthwhile to me.
I have never tried leetcode, though.
If the focus is in data and analysis I would recommend the exercises in the R for Data Science book from Hadley Wickham (https://r4ds.hadley.nz/). Even if they are simple exercises, they cover a lot. If the focus is in R programming and package development, then the Advanced R book also by Wickham, https://adv-r.hadley.nz/ has also exercises that come in handy as small tests and also cover all subjects.
Code Wars has R problems to work through.
Kaggle has data to work with and R learning resources