You can ask chatGTP to re-invent a brand new iteration of the wheel for you. 
A big part of what you are asking for is a feature sometimes called “Design blocks”. This would let you quickly switch between using break-out boards, and implementing the circuit and schematic of a breakout board in a project. But this is not implemented (yet) in KiCad.
It’s also not a golden bullet. Someone would still have to put all those breakout boards into KiCad projects. For the ubiquitous Chinese LM2575 clone board this is probably done relatively quickly, but there are a gazillion of those breakout boards “out there”, and quite a bunch are already “Open Source / Hardware” or trivial to reverse-engineer. This then makes it a “library management issue” for the most part. Such a library would also be a nice start for discovering “interesting” parts for hobbyists.
As far as I know there is no copyright on PCB layout, but I think that Paul Stoffregen would not be entirely happy if his Teensys ended up in such a library. As far as I know he does not publish the EDA project, but just the schematic and PCB pinout.
Another issue is PCB density. You have used THT parts for everything on your PCB, while using an IC such as on the Teensy 4 will require a quite dense SMT PCB and at least 4 layers, and this is quite a big difference.
But more to the point.
In your particular project, your PCB is quite fully packed. Apparently you want to make the PCB smaller, but changing the breakout boards to their individual components is not likely to make your PCB smaller.
A more logical way would be to change all your discrete parts to SMT, and then put them under the breakout boards, or put parts on both sides of the PCB. But dual sided parts placement is an extra complication for manufacturing too.
Another option is to trade size for height by putting the circuit on two PCB’s.
Also, you write you want to trade your time for money. PCB design is a bit complicated on the communication part. There are many tradeoffs which require communication, and thus require time (which costs money) from both parties. Outsourcing PCB Assembly is also quite expensive for small production runs, as a lot of time is needed for acquiring the parts and setting up the machines.
It may also be a case of “premature optimization”. How many of these boards do you expect to sell? If it’s below some 50 to 100 boards, then doing the soldering yourself is still doable and just keeping the breakout boards (or at least keep the “high density” Teensy) is probably the best option. Maybe you can find somebody locally to do the soldering for you.
For hand-soldering, I also prefer SMT parts to THT parts. You don’t have to turn the PCB, no parts falling off, you don’t have to clip resistor leads, etc. SOT-23 and SMT resistors are much easier and quicker to solder then TO92 and THT resistors. (Fine pitch IC’s and BGA’s are a … “different category”). But I do highly recommend a decent stereo microscope (A few hundred EUR) for this. A stereo microscope does not only make placement and soldering easier, but because you can see much better what you are doing, it also improves soldering quality, and of course also visual inspection.
