Why We Should Mostly Just Abolish Penitentiaries
Prisons are expensive, even as is. You're proposing making it even more expensive with more amenities with the expectation that people will stay there longer and be removed from the population of potential victims for long periods of time. I'd roughly guess a combination of longer time spent in prison, more people choosing prison, and increased amenities would double or triple the resources we currently put toward it, although I'm open to arguments that it'd be less (or more). I wouldn't reject this out of hand, but I have two points and a counterproposal.
1) You're talking about isolating violent criminals from the general population so they won't have victims. But an issue: they'd still have potential victims, in each other. One of the worst parts of existing prisons is the other prisoners. Even a prison with Michelin quality food and an onsite free spa is unappealing if you've got a good chance of being assaulted, raped, or murdered. Separating the violent from the rest is tricky, because most offenders can be violent depending on the context. And throwing half the prisoners into permanent isolation is a crueler system than what we have now.
2) Suppose someone stays for 5 years. What happens when they get out? They're in a worse place for integrating back into society than when they went in, except for aging out of the most violent class of offenders (adolescent and young adult men). Now, that caveat is a pretty huge one, in that if you removed all the criminal teenage and 20-something men from the general population you'd eliminate the large majority of crime. But they're still not likely to be able to reenter society meaningfully, and their net contribution to society over their lifespan will be negative including the cost of prison.
So what's the alternative? Get rid of prison terms of length 0-20 years entirely (the majority of them). These kinds of sentences rip people out of their communities, permanently disrupt their lives, and put them in an institution that allows them to professionalize their criminality. Instead, focus entirely on removing the risk of false negatives when it comes to decisions of whether to apply punishment, even at the expense of increased false positives. The deterrent effect of punishment has a lot more to do with the certainty of being applied than the actual severity of it. Replace prisons with corporal punishments, which are an order of magnitude or two cheaper to implement and don't permanently ruin lives. Then, put any guilty offender under strict parole (spanning the years when the criminal is most likely to reoffend), complete with ankle monitors and integrated microphones, enough to make it a certainty that any recidivism will be caught.
For severe cases (serial killers, DC shooter, etc), either life in prison (whether austere or luxe) or the death penalty, depending on your particular views on what's the most humane way to permanently remove them from society.