Psychron,
I made the mistake of reading this thread through just before going to bed last evening... and then spent half the night tossing and mulling it over. I'm a 73 year-old, tired/retired, electrical engineer, who introduced the needle cutter to the RC community a number of years ago, and who is often too outspoken and border-line senile... but what do I know? So, to keep this at least partially on-topic and answer your question...
YES... some of us have "figured out how to print directly to DTFB".
This is not intended to anger or offend but IMO you're "over-constraining" the solution to your problem and being a bit short-sighted with only wanting only to "dip your toe" into the CNC/automation "pond". There is no reason you couldn't be cranking out your own custom "speed-build kits", by the boatload... in relatively short order and for essentially the same $$$/time/effort you are willing to expend now for a much lesser "solution".
You want to BUY a machine to AUTOMATE the process of DRAWING/PRINTING (nothing more) plans directly to DTFB... which you will then CUT OUT MANUALLY. The truth is that, for essentially the same $$$ or less, you could easily have a machine that can not only DRAW on your foamboard... but CUT it as well. And, simultaneously, eliminate the need to DRAW your plans on the foamboard in the first place.
Further, and more difficult, you'd like this machine to be ready to go right out of the box, handle 20" x 30" foamboard sheets, have little/no learning curve, include all required hardware and software, have dead-simple controls that requires only minimal knowledge/effort to operate, and costs less than $300 or so. Otherwise, you feel...
"Ain't nobody got time for that."
Perhaps recognizing that machine doesn't exist -- at least, not off-the-shelf and in your price range -- you're now looking to spend $$$/time/effort on RE-PURPOSING and ADAPTING a vinyl cutter or printer to simply pen-plot on DTFB... and basically retrace the steps of the RascalCNC folks and others. That rarely, if ever, works as one would hope... and the RascalCNC's existence might well be proof of that.
What folks are trying to tell you... is that you could easily have a PURPOSE-BUILT machine that can do EXACTLY what you say you want to do... and FAR MORE. It will cost you a bit of well-spent time/effort to learn how to use your new tools but the reward is that you'd gain a quantum leap in capability and attain a few new skills that will serve you incredibly well for the remainder of your life and/or "building/flying career"; i.e. it's a "game-changer".
This more complete solution would save even more of that precious "build and fly" time on down the road... and virtually every build from here on out could start with essentially a custom "speed-build kit".
There are several relatively-inexpensive machines that can not only DRAW, but also CUT, your plans... all documented and in use, right now, in the FT community. And, as a result, there are plenty of friendly, helpful, experienced folks willing and able to help you get going... just a keyboard away. RascalCNC and MPCNC are both well-engineered machines that can easily do the job and are a relatively inexpensive, fun, and educational way to get into the CNC "game". And my current favorite... Mark "moebeast" Moebes, of FliteFest fame, designed and developed an even simpler, less expensive, machine called "Foam Ripper" that would IMHO be a RC scratchbuilder's dream
https://www.thingiverse.com/thing:2520214
https://www.flitetest.com/articles/another-cnc-foam-board-cutter
So, mount a pen/marker/dragknife/router/needle-cutter on any of these machines (the FoamRipper/router combo, probably not...) and you'll never look back. Can they DRAW? Yep... not a problem
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Can they CUT? Yep... and it eliminates the need to draw anything at all
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Yes, there are a couple of pretty steep learning curves... about on par with 3d printer hardware/software. Yes, these machines can become very addictive... in, and of, themselves. Yes, it takes a little time to CAD up a plane design -- and for a "one off" makes little/no sense (unless you just want to do it...) -- but you'll RECOUP YOUR TIME IN SPADES every single time you make COPIES and/or REPLACEMENTS.
But you're in control of that. And you're in control of whether you want take the time now to make the effort to get an education that yields a more complete "solution"... and greatly enhances the enjoyment level of your RC building future.
Again, I meant no offense by any of this. But I've been around long enough that sometimes you've just got to speak up and take your chances. Ignore me completely if you want... I'll not clutter up your thread anymore unless invited. And I apologize in advance if you feel I've been out of line with what I've presented. It's just that there is so much myth/misinformation/ignorance of what's possible nowadays with the incredible and inexpensive technology we now have at our disposal... and I felt it necessary to try to address some of your voiced apprehension and concerns.
Anyway, that's my $0.02.
-- David