Is this just a keyboard defect or more
At this point, you know you have a keyboard that's much easier to work on than the new butterfly design and an excellent / detailed explanation how that keyboard works.


Keyboard key stuck or not being recognized; how to fix


Also, you have an appreciation that doing this removal of a key has a very non-zero chance of breaking things. At this point you have a resource dilemma.


How much is your time worth?
How many parts and training do you need to successfully re-assemble the physical case?


If you want to fix it yourself, you can test all the keys physically by making sure the nubbin presses. If that's broken - you need a new top case or you need to consider using a bluetooth keyboard (if you must be portable) or a USB keyboard if low cost and low mobility is best.

Next cheapest is trying the top case repair, but you run the risk of under-estimating the tools and training needed there and could also break more than just the keyboard if you attempt a top case transplant of the computer (you're moving all the parts of the computer to a new frame, basically).

Also - consider getting a quote. Apple might charge $300 to repair everything you messed up and everything wrong with the computer functionally and put a 90 day warranty on all the parts touched in the repair. You're basically buying the best used Mac verified by Apple and you know the previous owner. I don't know many people that would turn down that good a deal if they needed a new computer, so don't look back about how cheap this could have been if the repair had only worked. It didn't (for reasons) and you're now faced with what next now that you know DIY repairs might have more risks than apparent at first. (Also don't undervalue your experience and knowledge you gained in this. I know some of the best repair people started just like you, so with time and training this may teach you more than if the key swap just went well.)