Form builder schema for SQL database?
It's doable and quite safe approach is


Store form definitions in form_type table with JSONB and proper indexing. The table should have a marker telling if the form is already SQL-ized with create table
Write a asynchronous processing tool (cron worker) which will update table structure and issue proper CREATE TABLE requests, marking form_typeas deployed to SQL 
form submit requests can be issued only after the form table is deployed
you can and should limit permissions / quota for this deployment task


Why async? I would avoid doing this in trigger due to locking issues under high load. Also, easy to DDOS so permissions should be separate for form insert and form create.