Hello friend I'm a full stack web developer with over a decade of experience, I'm also the author a popular shopify theme, and also the creator of a mockup and design system for customizable items. In short, I have the experience to finish and deliver quality to your project.
There are 2 main options here, but from your description I was unable to pick one. The options are:
1) If you own a separate server we can ask them for the file, send it to the server, generate a new one, send it back to shopify and shopify could then send the url as part of the order
2) If you want to leave it client side so the user experiments or use them elsewhere then that's possible too, but the limitation is they can download the file but it wouldn't be possible to submit to an order without uploading the resulting file manually
Meaning, method 1 requires a server but it can generate the image and send it as part of the order without having to download the image first
method 2 doesn't require a server, everything is done in the user's browser, but it has to be downloaded if you want to submit as part of the order
So, method 2 is better as a separate page, and more like a tool, method 1 is better if you want to automate the process.
Send me a message if you need more info!