There's already the possibility to have multiple settlers live in the same house, for plots to have multiple workers to have more production from a single plot and extras from one settlement can be shared via the caravan plots. At this point, there tools to make your settlements minimalists are there.
As for an all-in one training plot, maybe someone will make a mod like that but I still think it's not a great idea. It's not like you need all training plots in all settlements anyway. You want to train people on agility to get better result on martial plots? Create yourself a "police academy" settlement where you send people to be trained for agility. Once they are at the level you want them ship them to the settlement you want them at. And the same can be done with every other training plots.
As part of the reasons for their design decisions, one of the things they wanted was to have settlements differ from one another. So that you can have an agricultural settlement that over produces and send it to settlements that are industrial themed (or however you want to build them) and that settlement could send items to a commercial type settlement etc. What they wanted to avoid was that every settlement becomes a carbon copy of the previous one because "each settlement needs agricultural, water, power etc". Hell with the power transfer plots even electricity can be created in one place and shared, no matter how many laws pf physics it breaks to have electricity be transfered magically without wires over long distances. My point is: An all-in-one training plot would become the only plot people would build in all their settlements and then we're right back to every settlements are the same.
As for needing to spam plots, I have no idea why you would do that but yeah, if you spam plots without reason you will most likely see issues with your performance. Don't do that, use settings that are gentler on your machine (ex: turn off visitors) and build plots intelligently to limit the strain or your system. For instance, maybe a 3x3 agriculture that allows for 3 settlers to work could be less of a strain than two normal agriculture one while giving the same amount of food.
Oh and mybe I'm wrong with this one but my impression was that interior plots were a lot more gentle on my machine. I was able to put residential interior plots in all the sanctuary houses, multiple plots per house and got to 15 or 16 settlers and my old xbox is still fine with it. But when I tried the same amount of 2x2 my machine was really not happy with me. Again, maybe that was just an impression on my part, people who know the system more might be able to say if I'm right or completely off.
