If you get a /48 you can probably evade the problem by assigning a /64 for your proxy at a time. You will have another ~65,500 such blocks for use.
Yes, some might just block the /56 (you would still have another ~250 chances) or /48 but nothing is perfect.
but how do I change the IPv6 address in AWS? I can just assign a new Static IP for IPv4
Same thing for IPv6, but instead of assigning a new singular IPv6, you assign a new subnet with millions of IPv6 addresses.