I am by no means an expert in any of this, but I have spent the last 6 months practicing wheelies and manuals non-stop, so it's still front of mind. YMMV.
You might actually be sitting too close to the tyre and/or not standing up enough. This was certainly the case for me. Even after I (just quietly) became a wheelie pro, I still couldn't manual for more than 10m without going over sideways. All the videos tell you to do the reverse L and get low and back, but I personally found that gave me less leverage over the bike and no matter what I did I couldn't correct it. You might be the same.
Try this...
Instead of trying to manual by rolling along with your seat slammed like the videos tell you, try transitioning from a seated pedaling wheelie into a seated rolling wheelie and think about where your body is. Find a gentle slope and wheelie down it then stop pedaling.
Even for wheelies, the videos tell you to drop your seat but I don't think that applies to everyone. I am 6' riding a 29er and can hold a wheelie for 500m+
until I lose concentration or my hands cramp up. For huge wheelies, I am only dropping my seat 1" from full extension and I am also sliding forward on the seat (higher vertically in wheelie mode). When I manual, I am only slightly further rearward to counter the input from the pedaling force, but no where near as low or rearward as the tutorials would suggest I should be .
I also think that the knees are a secondary motion. I balance with my arse/hips and I think my knees stick out and move around as a consequence of the hip movement, I certainly don't make an conscious knee movements.