Given your lack of experience as a gymnastics coach and teacher, it's no surprise you think you know more as a parent about what is right for your spawn than the professionals do.  This is typical of parents with the god complex, they figure despite not knowing the first thing about children, they are instant experts just because they forgot to wear a rubber one night and popped one out of the oven 9 months later.  Even the ones who actually plan it know little to nothing, which is a big part of the reason we have so many fucked up kids in the schools who turn into fucked up adults who make still more fucked up kids.  We have gone through numerous cycles of this stupidity, leading us to the fucked up society we have today.  As you might expect, I am all in favor of Boarding Schools. lol.

Why did I never propose this idea during the SUN years?  Well, first off I didn't think of it back then, second it's too expensive except for a Billionaire who would put a significant portion of his personal wealth towards doing it.  Unlike building rockets which have the commercial application of putting satellites in orbit, there really is no commercial application for this or profit in it.  The only way you could make money off it is rather than populating it the way I suggest by competitive examination would be by selling spots to the moderately rich.  If a small Bubble Town for 1000 people cost say $5B to build, it would cost $5M/person to buy your spot.  Problem here is any idiot with $5M could buy a spot, so you don't get people with the right skills and abilities populating it.

It would of course need some defensive weaponry,but given its size and the fact it would have around a 5 foot thick basalt reinforced concrete shell it would be quite impervious to even the pirates who go after supertankers.  You would need a battleship with artillery shells to blow a hole through it, and that's only on the surface.  If you submerge it, you would need a submarine with torpedos to blow through it.  In either case, you would likely sink it, so what's the point?  Just to kill the people who live there?  It's not a source of resources.  Anybody still in control of warships capable of sinking it would be living just fine in armored bunkers on land.  Eventually the various countries competing for resources will sink each other's hardware and run out of fuel and ammo for their warships, and this level of threat will diminish.

As you mention, you are never going back to braining raccoons and I will never be boarding a 2000' diameter nuclear powered floating concrete egg, so these are just thought experiments and philosophical arguments.  Similar to arguments about the existence of God, nothing is ever proved other than the fact we  have diametrically opposite differences of opinion on a variety of topics.  That's what makes the world go round.

RE