This is the common problem.
None of the remedies described above help much.

1. You need a kayak with higher deck to let you bend legs at the knees more.
It might not be much better, because with higher deck you might get a
real clunker - big and heavy expedition style kayak.

2. Another option is "open cockpit" racing kayak like West Side Boat Shop
or Nelo. You probably never heard of them.
These are fast and tippy (18" wide) kayaks - not good for a beginner.

3. Surf ski is another knee - friendly craft. They are "sit on top" style.
Those are even tippier than kayaks.

Kayaks were invented by generally small people for specific tasks
and do not suit all the people.

Modern forward paddling stroke technique generally requires you to
seat with your knees up and torso unobstructed for rotation so
most of the kayaks that would feel comfortable for you are built
for racing.

I had the same problem and abandoned "sit in" kayaks if favor of
surf ski and never felt any numbness or even tension ever since.