Coldfusion 2018 Migration Guide

There is nothing official from Adobe on a migration across so many versions (and I'd say, understandably, though I appreciate your position).

And no, the compat checker only ever goes back 2 or 3 releases, if I recall correctly. I've seen Adobe folks recommend that someone making a jump from such an old release could implement a version in-between, so in your case, cf11, and use its compatibility checker. That seems a hassle, but you can find old CF installers at cfmlrepo.com. I've frankly not ever found the tool to be particularly valuable, often mis-reporting things, but ymmv. 

Better still, you can find that others have written on/blogged their migration efforts, and sometimes documenting it in some detail. The challenge is gathering those (something I'd like to do some day, to help folks like you).

In the meantime, I will say first that for many, it's not a large effort--yes, even from 9 to 2018. There may be some small number of annoyances, or none at all. Or it may seem the "whole site fails", but the problem is in the application.cfc/cfm, such that fixing one things then makes "all those failing pages" able to run--at least until you hit perhaps some other issue specific to that page. That may be painful, yes, but really not much you can do otherwise--and again.

And some people may well find that some change was so devastatingly different that they feel it's "impossible" to migrate. I will say I've only heard stories of people giving up like that. As I do CF troubleshooting, and have helped people move even from CF7 (!) to CF2018, I can tell you that there's not been any problem someone had that I could not help them get around--and none were ever "so scattered through the code" that it was "impossible" to migrate.

It's just a process one must go through. It may be tedious. It may take minutes or hours, or days or weeks. But really, it should not be as daunting as so many make it out to be. Adobe has always strived to keep CFML backward-compatible, and indeed to a fault in the opinion of many.

So I leave you this encouragement--not just that it may not be difficult, but again that you can find resources where people have documented the process, and that you CAN implement an intermediate version to use the compat checker if you want.

If you may give it a go, I hope you may think to come back and let folks know how it went, whether good or bad.