allow unregistered users (customers) to upload files

Hi, I am trying to find the best solution for my customers so they can upload files through my website. Ideally the user could attach the file(s) in a form when they ask for a quote. I could use my contact form plugin but I am hesitant to store the files on the website server. Most of the files will be drawings with uncommon file extensions typically enclosed in a zip file. I need to allow zip files to be uploaded. I looked into some plugins that allow you to upload to the cloud and the only one I really found was wp-file-upload currently there is an issue with the Avada fusion builder so I am looking for another solution. I also found filestack but I’m not wordpress savy enough to put the link inside a form. Looking for any suggestions, just want to keep it simple for the customers.

Some of the requirements:

send large files (under 250mb will be common)
guest uploads (no signup for a service)
safe files storage (wont let my website get hacked)
secure files after upload (can only be downloaded by myself)
allow zip files
upload via form submit if possible
plugin if possible, not good with code

Should I be concerened about storing the files on my siteground server? any suggestions are welcome!
Thank you for your time