[Wheelhouse]

Try taking the plywood cover off the bed frame. Now attach a guide running board for your router on the outside face of frame.
This eliminates tearing the frame apart and rebuilding it.
Adjust the guide board the amount you want to lower plus the router plate. You will lose some or all the storage of the matress hidden storage.
Now do the same to the headboard ledger. This if you can removal would be easier for cutting.
After you finish cutting with the router, finish cutting with handsaw or other type.
Now replace plywood cover and matress and you should be set to go.
Hope this helps.