I believe it will fit if the 2021 dimensions are the same as the 2023 dimensions. I did a post showing the amount of luggage space available in the CX5 when I was looking to buy one (I did buy a 2024).

https://www.reddit.com/r/CX5/comments/14i0zer/another_debunking_post_the_amount_of_cargo_space/

The Samsonite hard shell suitcases are 20" wide and two of them can fit fine side by side in the trunk.