Intermittent remote freeze w/ Streaming Stick 4k

It's hard to say where exactly the problem lies, but there are a few things you can try.  The extender was a good start and if I'm reading you correctly it sounds like the situation improved after you got one.  I've found that the remotes can be finicky when it comes to batteries, so trying good, fresh batteries would be worth a shot.  The sticks also support 2.4 and 5GHz Wi-Fi.  The remote works on whatever channel the stick is using for its Wi-Fi connection, so there's lots that you can play with there.  First, if your router's Wi-Fi channel is set to "Auto", set it to a fixed channel.  That alone may help.  If you have both 2.4 and 5GHz available, try both of them.  Try different channels on each.  Bear in mind that only channels 1 through 11 are supported on 2.4GHz (standard for US) and non-DFS channels are supported on 5GHz (48 and below or 149 and above).