This is long....
I was in the same situation looking to hold a 12.9 iPad Pro 5th gen which is pretty heavy. I looked at all the mounts on amazon and they were either plastic or utilized the existing mount meant for a phone which relied on the spring loaded antenna arms. Most folks are ok utilizing that method but I did not want to damage the arms given the mount will extend out creating additional weight due to leverage. Plus it seems over time, the plastic pivots wear and you have to buy another unless you use the extension and again straining the antenna arms.
I looked at the LifThor Baldur mount and that looks well built but seems to be more of a permanent mount which can't be removed and reinstalled quickly plus if left on will no longer fit many form fitted hard cases which I have, or most bags for that matter. Not sure what material the holder is made of (the part that actually holds the tablet) if that is plastic or aluminum, maybe someone who owns one will chime in. I can't tell from picture and I am not sure how tight you can get that pivot ball if the assemble is plastic.
I then looked at the Mavmount Scout Prime 122-235mm (4.8″-9.25″), easy to install and remove, does not rely on the antenna to mount but more the inside area of case. This is the mount I bought but there are issues with this too. As shown in the circled 1st picture (in my case at least) scuffs the controller. You can remedy this with some clear tape. The second issue I have is the mount with the heavy 12.9 iPad moves a little. In the second picture I circled TWO pivot points which is pointless. The bottom pivot can be tightened and is made out of aluminum so no issues there and they should have stopped here. The top pivot point is part of the tablet holder and is made of plastic with a metal ball which can be tighten only so much before the threads bottom out plus it's plastic and can break. Doing this does hold it tight but will move a little bit due to the size of tablet. The last picture show how that ball assembly attaches to the tablet mount. I don't like it, seems a little weak for the weight of the12.9. I guess I am a little paranoid about mounting the 12.9 iPad given the cost was slightly over $1,500. So I purchased a different tablet holder,
Full Aluminum Mount to replace the plastic one. This is completely aluminum except for the wings on the wing nut at the back. I like that is has offset holes for mounting plus this is tightened via the wing nut and the top micro adjustment screw not springs but you need to make sure not to over tighten it. I have test this by mounting the 12.9 iPad Pro and shaking it upside down (over a pillow), did not move one bit. Another option but center mount only
Aluminum mount.
So I spent a little over $100 (73.99 on sale Mavmount Scout plus 30 for the full aluminum mount) on a tablet holder but given the options on amazon and the cost of replacing those when they wear out, plus my paranoia of mounting a heavy $1,500 tablet on it made it an easy choice.
You have the 11 iPad Pro, not cheap, should be an easy decision...