Should I try everything the same, but send the string without base64 encoding?
The string being sent, if you can read it, is NOT base64 encoded. Whether that happens because the phone app decodes it before sending, or for some other reason, I do not know.
With all your arrays, how much SRAM are you using? Do the arrays NEED to be that big?
What I would do is comment out all the code that USES the data, and just print the values parsed. Send the data as fast as possible. If that data all gets through, then it is the process of using the data that is causing problems. If not, then it is the process of sending or reading the data that is causing problems.
When we know which problem you are trying to solve, we can solve it faster.