Originally Posted by Papa-T
On my 2016 5er (queen bed), it was the same problem. We replaced our cheep mattress. The new mattress hung over the platform at the bottom about 12”. I removed the mattress, replaced the lower platform with a longer piece of plywood. I used the old platform to mark the width but cut the plywood 12” longer. Basically your feet are in that area and there’s not a lot of weight there. Had to replace the hinges and shocks to reinstall. Made for a much comfortable bed.
I considered installing a longer piece of plywood as you did, but didn't think it would be sturdy enough to support much weight extended out over the end of the bed. That's why I went with the bigger dimensional lumber (plus it was an easier to go that way versus redoing all the hinges and struts).
I also bought a 4" thick Blue Swirl memory foam mattress topper to fix the deficiencies of the factory installed mattress. But I use it under the mattress. Works best for us that way.