Yes, that seems to be the smartest idea. Was just hoping you could get what you already had on you working. I really think it is the size more so than the brand.
This is Roku's product page. https://www.roku.com/products/accessories/player#item-SD02
Note near the bottom they recommend a 2gb card for the Roku3. But as you have seen with your own research, end users can get bigger cards and different brands to work but you have to do some fiddling. I did not see anything where it says "up to" or max specs. Since users have 32gb reportedly working, I think that is probably the max.
That is why it can get irritating when everyone just points to the Roku's own support for the troubleshooting, or a "canned" response like it is the final answer. It may be the recommended method, but it surely is not the only method. (of course, if you deter from the recommended method, it is up to you to make it work).
I think we can almost definitively say a Sandisk 64gb class 10 micro SD will not work with the Roku3.
Unfortunate you couldn't it get to work. Post back what you do get to work so maybe it will help some other users in the future with same problem. Good luck!
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941