What I would do, first of all, is to recognize that “this is not a new problem.”   It has been solved before.   And the way that it has been solved, in each of its various ways, is to assume the existence of some kind of “batch processing” system, independent of the web-site and of the web-server, to which units of work can be “submitted.”   The web-site then becomes a user interface to that system ... providing the end-user with the ability to submit these units of work, and to inquire as to their completion status, and to retrieve and view the results obtained.

What I suggest that you do next is ... on-line research.   (Which is a good bit more than just, “Google™ It.”)   Basically, you know right now that you ought not to have to build something new; that you can either buy or obtain an appropriate (and well-tested) solution much less expensively.   But what you do not yet know right now is:   what are the alternatives that might be relevant to my problem, and, of these alternatives, which one should we choose (and why?).   Only good, old-fashioned, “gumshoe research” can tell you that.   (This is not a “blow-off.”)

Actum Ne Agas:   Do Not Do A Thing Already Done.