You just had very poor mash efficiency.
All the causes and solutions have been covered in other Q&A here.
When done correctly you should be able to easily produce 10l of 1.096 wort post boil and dilute to 20l for 1.046
Update: Ran some of your numbers. DP fine, water/grist ratio fine. Looks like you actually got ok mash efficiency but saccharification may not have completed. Your overall efficiency was also dropped by the grain absorption (5 liters) and the sparge/rinse method wasn't effective.
Moving forward: Iodine check the mash for completion rather than a timer.
Suspend the grain bag above the pot to let drain. Then put the grain bag in a strainer bowl above the pot, pour 168F water through the grain into the pot to get 10l total wort volume. Squeeze the grain bag to get the grain dry as possibe. This should get you close to the 1.096 10l target.