Wordpress Plugin Development (or API) For Existing Web Apps

I am looking to hire multiple WordPress developers to help with the building of plugins/api wrappers for existing web applications. Main Goal: Tether/Bind these standalone web applications (each also has an API) to Wordpress users who sign up. Proposal Requirements: How long will it take to create each plugin/api wrapper (use the example below). How much for each plugin/wrapper for each app? (The $750-$1500 is based on 4 wrappers). First off, I am open to hearing other possibilities to execute this - but the best solution I have come up with is an api wrapper/wp-plugin wrapper around the stand alone web apps. For example - I own a copy of BuilderJS ([login to view URL]). I am using Wordpress to handle user management/authentication/payment. I need to be able to embed this newsletter builder (builderJS) inside - and bind it to my users. This can either be done by creating plugin or by API wrapper (very similar approach). I am ok with leveraging services like ([login to view URL] , [login to view URL] , [login to view URL], [login to view URL] etc). User Flow Overview: - User signs up on my SaaS website -- Account creation and billing handled by Wordpress --- Wordpress Multi Site created (subdomain - eg. '[login to view URL]') - In their wordpress admin panel, they have access to their instance of BuilderJS (for building newsletters and exporting them) --- Additionally in the BuilderJS app, they can already 'upload' images - but the images need to be able to save to the "media library" for reuse (the app is capable of this already by directing where things need to be saved - see documentation here [login to view URL] -> click "help" in the menu bar to see developer documentation reference -- Save & Export features need to be setup as well (and binded to user) ----- You can see an example of the 'theoretical setup' I am talking about here ([login to view URL]) - where a static menu bar (wpadmin menu) is framing the admin section of WP. For reference - where the 'gutenberg editor' is now is where I need BuilderJS to go. I do not need the admin entire section customized or built out - just the ability to "plug" builderjs into the same place. ----- I am looking to do this with multiple web app - so I am looking to hire multiple WP freelancer to work on this concurrently. *** I am looking for fast turnaround time, ability to meet deadlines, and good communication. Would ideally like to develop a part-time/full-time freelance relationship because of the number of backlog WP tasks and projects I have at any given time. ***