Best way to stream base64 encoded string to a web client

I personally split html in logical parts like head and body as needed and use this style to keep easy to read.

s.write('html',`

........

`);

I have no experience with large data amounts you are trying to handle, max was 3k.

Also can I upload the .bootcde script

see E.setBootCode() and Boot process: (v2.0 and later) It looks for files in Storage named .boot0, .boot1, .boot2 and .boot3 and executes them in sequence.