The backplate helps with sagging, but these cards are so huge it would be difficult to not have them sag a little bit. Mine wasn't terrible, but I ziptied the power cables to the hardrive cage to hold it up. I read of someone else putting rubber caps on the end of a dowel rod that was cut to the right length to prop it up and another guy using black lego blocks to hold it up.
After a certain age sagging becomes inevitable....I don't know if ASUS make a Strix push up bra 😉
Ahem...seriously...screwing the two points to the case should help a lot as Nate says...or rearranging the power connections so the cables are helping to pull up instead of down...like top card here not bottom card
Clay's support rod looks smart ... wood works well but I would've probably used a plastic.
Another PCIe bracket (like this or this) might help.
Use metal slot screws. Avoid using any plastic slot screws/clamps which might've come with the chassis.
Sagging would be a non-issue with horizontal motherboard, lol.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams [/Korth]