I also can do full lotus, but prefer to meditate in half lotus. The half lotus is less constrictive than the full lotus. In addition, elevation of the torso helps decompress the legs, so a standard zabuton setup can help, although I’ve had to take out some cushioning material to flatten the round cushion a bit. Too high is just as bad as too low.
Numbness is always a concern, especially if there is any force involved causing the numbness:
DN34:1.6.73: ‘This immersion is peaceful and sublime and tranquil and unified, not held in place by forceful suppression.’
Some traditions such as Zen can impose time limits of 30m or even longer on practitioners. Misunderstanding these conventions as mandates can be dangerous to health, especially if nerves are being squished. I actually have a pinched nerve issue that I have to work around, so numbness is a bad sign.
Reading the suttas carefully can help here. For example, numbness can be understood as a form of dullness. Dullness is a hindrance, and therefore numbness should be avoided as well. Read the whole of MN128 to get a sense of the scope of issues. Immersion is very subtle and a lot can go awry.
MN128:18.3: ‘Dullness and drowsiness arose in me …
In particular, note that although the Buddha meditated a lot, he gave no specific time mandate (e.g., 30 minutes). Instead, the Buddha simply encourages seclusion and meditation:
MN8:17.3: Here are these roots of trees, and here are these empty huts. Practice absorption, Cunda! Don’t be negligent! Don’t regret it later! This is my instruction.”
Time mandates are a modern imposition brought about by things like the demand to punch in work hours. Shifting away from those external demands and assumptions, we can focus on something more immediate and real. We can count our breaths, for example. And if you count your breaths mindful of the body and your feet, you may notice when your feet start to numb. It will be subtle, but becoming aware of that transition may help adjust your practice to be free of that numbness.
One thing that became quite apparent to me in reading the suttas is that the immersive experience is not restricted to sitting. Sitting is beneficial for depth, but breadth is also important and immersion can permeate our entire lives.
AN8.63:10.1: When this immersion is well developed and cultivated in this way, wherever you walk, you’ll walk comfortably. Wherever you stand, you’ll stand comfortably. Wherever you sit, you’ll sit comfortably. Wherever you lie down, you’ll lie down comfortably.”