help for upgrade codeigniter3 to codeigniter4 in imageoptimizer project

Hi,

For making uploading to use AJAX technology you can use OpenSource project Dropzone.

But...
You will essentially reinvent the wheel, since such projects have been done for a long time and are even open source. For example: Compressor.js

For CodeIgniter does not matter what protocol is used to communicate with the client's browser (HTTP or HTTPS), this task is for the HTTP server. And a free certificate for it can be easily obtained in Let's Encrypt
All this is mainly UI development using JS and servers administration.

And I only do Backend and CodeIgniter in particular, so I will can not help you the development of such a project.