Website Software Development (SaaS)

Please note that I have a fully operational wordpress version of my website service to use as a baseline example for functionality and overall concept.

Website Concept:
Members login with their own user credentials and are then given access to interact with their own area or portal, only.
The web page is a singe page with a tab style menu system as the page body.
The menu is used to preset the user with buttons for printing timestamp, custom worded labels that print on a zd-410 zebra brand label printer.

The problem:
I am not able to scale my offering because:
1) Wordpress plugins are a nightmare to keep up-to-date, and I don't want to trust all the plugins when I scale this up.
2) I am don't know how to make one url that all users login to, and so i currently create a new subdomain for each user under a wordpress multisite configuration.
3) I need the ability for the labels to be made by the user because currently I code the labels as a container that will print based on a ID tag.

The goal:
1) Use the wordpress site i currently have to provide a developer with exactly what needs to be created, but on a different platform
2) Develop additional functionality into the site to make it more admin friendly and more user friendly
3) Help me understand an choose the web service to build on (my current web host, or AWS etc...) and what technology to build with (react native, .net, etc..)